Maybe cold-flashing will revive it (it does not erase the data, just flashes the 1st-stage bootloader)

Nope, but if I put the battery very slightly into the device, it shows up in device manager for 2-3 seconds and then dissapears.

EDIT: I guess it's something with the USB, when I unplug the USB cable from the PC for a few seconds and then plug it back in, the yellow light flashes quickly for 500 miliseconds or something and then stops, DevMgr says "Device Nokia USB ROM Disconnected" or something like that.

Try
another usb cable
Another usb port

Could be also broken USB port of the device. Maybe soldering is loose and it just makes momentary contact when you push cable in and then loses it again.

Or also 'broken'' drivers on Windoze.
Try reinstalling 'connectivity drivers' either by reinstalling flasher or suite.

Best would be to use LINUX OS and try once more (or just completely other Windows hardware).

Charging a few batteries to test, will report back soon.

HOLY **** GUYS! IT ACTUALLY WORKS AGAIN!

Thought it hanged for a second at the 5 dots, but it works now!

Sadly, I seem to have overwritten the data that was on it at some point, I only could find stuff from 2013?

Good!

Would be nice to know what you have done ...

Cold flashed it, I guess the bootloader somehow corrupted itself back then.

At least I got some of my personal data back.

So last monday it also happen to my heavily used scratched and dirty N900. I had it in my pocket and when I get it out it was off and would not switch on.
Because of a weak battery I thought maybe I got a call and due to high power consumption the phone switched off.

Symptoms:

• phone would not switch on no white led completely dead.
• connected to car charger the led constantly green. Not booting into charge mode
• tried different batteries charged in other N900 no effect still dead.
• connected usb to windows and linux system, phone was not recognized (dmesg, syslock) led stayed dark - no charging
• while connected to usb port removing the battery the led was constantly green. Putting back the battery led was still green.

Fix:

I opend the phone to look at the components if they are not loose.
I recognized lots of dirt, dust and organic mix.
Cleaning the two hot spots of dirt with ethanol and a small brush from a razor was the solution.
I marked the position in the picture below. The section under the hf shielding was so dirty that you couldn't see the electrical components.

It is still dirty and needs more cleaning. However it is already a difference like between day and night and it is working again.