I think it is an issue with some Li-on batteries that when they are totally dead they need a bit of a charge to get them to wake up.

One way, (and I'm in no way saying you should do this!!) is to try and put a surge of juice through the battery. This can be achieved by plugging the nokia charger into a wall socket with an on-off switch, and rapidly turning it on and off, while the device is connected obviously.

Iv managed to wake up a few batteries this way, but not ever tried it on the N900.

Please bear in mind that there is always a risk of frying your device, the charger and yourself. Please be careful playing around with electricity!