Not sure about a completely dead N810,...but I've been getting the main blue Nokia on white background loadup screen freeze then reboot for a long time too. Two things I do which together frequently works. First do as said before and remove battery and hold down startup button drains excess power. Replace battery,...do a normal startup,...but when the blue Nokia appears press and hold the function button while pressing the return button, then release. It will reboot again in a few seconds only this time it should take you to the two hands screen and then to the desktop. Good luck.

Excellent thread,

Much appreciated. I thought my device was bricked and I just resurrected it.
The knowledge of those who post here is outstanding.

Mine did the same thing yesterday and I have had it only for about three months. I tried several things and then called Nokia. The tech suggested a few things but when he learned that connecting it with the USB cable did not result in the device being recognized, he said the unit needed to come back to Nokia. Therefore, he is mailing me the instructions for return. I will post the results after action is taken.