Good.Your N810 was not dead.

1.Make sure battery is full.
2.install update wizard to your computer.You can find it at nokia web site.
3.follow the instruction step by step.
4.one important thing:you need hold power on and swap button till next button enabled.
5.flash your system.
6.format your SD card.
7.don't put too many files on your card like images files.

I had the same problem before.
Good luck.