I had a similar problem. I called Nokia and they told me to send it for repairs. I was not fully convinced that it was any hardware issue.

This time Problem happened within an hour or so of my last tuning it off with batteries almost full, thus batteries should not have drained fully even if processor kept consuming it. After Reading through this thread I tried just taking the battery out. I did not charge it. I just left it out for half hour or so and then reinstalled it.

Voila, N800 powered up with battery showing up as full. Thus I am thinking that battery was never really fully drained. But some process is stuck not allowing it to power up.

Someone else with this problem can confirm please?