I took out my N810 after a long time and put it to charge. After sometime the device turned on and has been on a boot loop since then. It shows the boot screen and then reboots. I left it to charge for a few hours but nothing changed. To prevent it from rebooting, I booted it up in Flash Mode by holding down the swap while powering it on. The screen shows the USB icon and stays on as long as I put it in charge but as soon as I disconnect USB it goes back to the same old loop.

I tried flashing but Flasher-3.5 fails to install on Mac OS X 10.11. I tried 770Flasher but it seems to be built for PowerPC and not updated since. I tried flashing using a Windows 7 machine but the Flasher is not able to detect the device.

If you guys have any idea how to fix this, kindly help me out.

with windows detection..
libusb is needed to install on your win 7 pc for it to recognize the connected cable and device...
3.5 flasher issues with win and n800 or n810?
use the nokia tablet update wizard utility with windows...

In total...
your probs are solvable ...
I suggest the walkthroughs in my signature...

It is possible that you may immediately solve this by battery removal for sayyyy a few minutes...
then try powering up normally...
but I suggest the walkthroughs even if you are successful ...
it'll clean the sucker out and set it up fresh...

Thanks! I will start with libusb then.
The issue is with n810.
Also everywhere it is suggest to have the device fully charged before flashing. I have plugged the device in for a few hours over the last two days but due to the continuous reboots I am not sure about the battery condition. Is it safe to flash in this condition?

the process is the same for n810 or n800 give or take a couple minute differences.
reboots do take a chunk out of the charge each time...yes.
Concerned about the battery condition?
Unless the battery has less than a charge capacity of 2 hours...
you should be able to get past the literal 2 minute "flashing stage" where being unplugged from a power source is necessary.
Then plug it back in and carry on with getting it snappy again.

Suggestion is.
Charge it.
Literally to the point the device says "battery full"
unplug the power cable.
Immediately if not sooner (don't want the battery now sitting for days...especially if there is a prob with it keeping a long term charge..)
Prepare your pc.
install libusb properly on the win7
install the nokia tablet update wizard properly on the win7.
connect the device and attempt flashing.
If there is a problem it will be with an omitted step in preparing the pc properly.

Essentially ...follow the walkthroughs or vid tutorials

OK so I do not own an N8x0 but the above screams to me, "bad battery!" Espetially given the background, the device being not used for a long time (years?).

The battery may not be charging at all. The device starts, drains whatever little juice is there left, and reboots. If the battery is really bad, it may never charge. I would start by trying to charge it externally (out of the phone, with an external charger if you have one), to confirm it boots at all. If I am right, the battery won't hold the charge for more than a few minutes, but it should be enough to confirm the theory. Once confirmed, look around for a new battery.

If you do not have a universal external charger but you have a digital camera or another gadget with a Lithium battery and a charger, you can fashion a makeshift charger using that and a couple of wires. I have even used some rolled up aluminium foil from a chocolate wrapping for that purpose in the past. But be careful, you may cause serious harm if you do not know what you are doing.

Thank you very much. I had been fiddling with this for more than a day unable to find a proper solution. I had tried to download Nokia Internet Tablet Software Update Wizard, but all the links were pointing to now defunct Nokia tablets dev site. I was able to find a link from a site provided in your signature. Installed Nokia Tablet Software Update Wizard and flashed the device using it. After a quick flashing I was able to boot up the device and the battery is full.

I hadn't used it for a few months. Even I had the same concern but I couldn't find another Nokia device that uses the same battery or an external charger to charge the battery and try. Anyways Nokia Tablet Software Update Wizard did the trick. After flashing the device shows full battery when I booted it up.

Long back I remember flashing the N810 using Flasher-3.5. I also enabled the RD mode. Now I don't remember how I got it working. I don't remember if it was on Windows 7 or Mac OS X(10.10 or 10.11).

Is there a x64 version of Flasher that will work with OS X 10.11 or Windows 7 ?
If not, is source code for Flasher or 770Flasher available anywhere to port these to work on 64bit systems?

Pichlo's assessment is sound and well documented here concerning dead or nearly dead batteries.
Although after reading...that your n810 was caught in a looping reboot...
I figured it wasn't about it being a dead batt issue.
Battery consumption is bad with every restart...losing a good chunk of time from your battery.
Hence you wouldn't be caught in a reboot loop on a n810 for very long...if indeed it were a dead or nearly ..battery.
It appeared more like the system needed a fresh clean and set up.

If the problems persisted after doing or attempting to do a proper reflash...then we would have to track down the culprit issue(s)

you can play with 3.5 or the 770 (although I do not suggest or advise in any way utilizing a method or tools not documented or meant for your specific device)
otherwise the nokia tablet update wizard works on x64 (just have to do it properly)
I would suggest either linux (a compatible and easy...32bit buntu or debian 32bit) or win xp or 7 (32bit is preferable ..but basic functional need should suffice if using 64bit) for as problem free a solution to the pc end of "what distro should I use?" topic..

As I have mentioned...
and I don't mind reiterating
For those who are new to the devices or those who have forgotten...
even I forget...
the walkthrough and the durability of these devices attest to the fact that you can let them sit for years without maintaining them ...
and they still work...
marvelous..
.but yeah...as a consequence I HAD to do the walkthroughs and videos...
if not for everyone here with a n8x0 .....
then at least for myself!
While I could remember! ...
since given enough time without doing the process...
I'd be in the same boat as everyone else!
screamin' "How the Hell do I fix _ _ _ _?!?!?"
"I can't remember!!!"

And over the years you would not believe how many times I forgot how to do the process..either in smaller or greater part...
only to go through countless pages here at tmo to remember how to do it all over again..

Got sick of that...
So the last time it happened (that I forgot how to tie my own maemoan shoelaces ..) ...
I vowed that WHILE I was doing the complete process I would document it, write it down as I went, step by step...
everything needed to make the n8x0 perfectly set up at it's optimum performance...
the easiest and simplest way possible ...
with the least hassle, confusion or complication...
in other words...
make it beginner friendly
(as in "a rutabaga on life support could do it" friendly)...
and make a video tutorial.
Now I refer to the past genius of myself ...
since braintardation has set in and I'm getting lazy
The walkthrough makes it so easy....inevitably it becomes a thoughtless, mindless automaton process and voila!
you are done!
That is how easy and familiar new people will become with the process...
So thank goodness there is the walkthrough.
And as well...curse you walkthrough!
The walkthrough is so easy...it is easy to forget...
hence the walkthrough is pretty important!

So yeah...the walkthrough isn't there for just new people..
.it is there for me too.