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.