@Benson: I left it this night on charger for more than 6-7 hours. I'm not sure about battery...so sudden drop and no blue LED light on pressing on/off button. I don't have spare battery to test it.

@Mara: Yes if I just connect it to PC it didn't showing up. But if I press on button.
The messages are:
Device inserted:
Nokia BB5 ADL Loader USB Generic

USB Device inserted:
Nokia BB5 ADL Loader USB Phone Parent

and right after them:

USB device unplugged
Nokia BB5 ADL Loader USB Phone Parent

Device unplugged:
Nokia BB5 ADL Loader USB Generic

this repeats every 25 seconds...
It didn't loop forever...it cycle for few minutes...and then stop to pop-up this USB messages...

Probably I can reflash again.... but this isn't solving anything...
Well....I should get my torx screwdrivers again and open it to check for some easy to spot problems. Duh... my Nokia phone (5610MusicXpress) was broken a month ago (distorted image)...and I decide to try to repair it myself....I ordered a new LCD, found a two movies how to dis/assembly it and it turns out that the problem was the LCD itself, not flex cable...so I replaced it and it's working now. I don't know if someone sell parts for Nokia N810 on eBay? I found none?
So even I found the problem....is there chance for ordering parts for this beauty?

P.S: @Mara: Thank you for the instructions....this night is mine....wish me luck... ;-)