I decided to switch off my Nokia N900 while going into some meeting room, but it refused to switch back on.
I assumed initially that battery might be low or dead so decided to charge but even that did not make any difference. Pressing power button would not have any effect at all! No charge light came on.
After lot of fruitless searching, I decided to take battery out and put it back and that resolved it!
Although some had mentioned that, In the manual it says if your battery runs very flat, you may need to plug the charger in. remove it and then plug it in again before it will charge properly but it did not work me.
Just go ahead and take the battery out and put back in.
Hope that helps.