'framebuffer' output could help here.

But as you surely do not have it enabled we need to guess.
juiceme's proposal is quite good but how to find one with a N900 (there are not so much Nxxx freaks around, at least here ).
Probably get a external charger (quite cheap: universal ones from China, links here on TMO).

It could really be a flat/dead battery ( after months of low discharging ) that even has not enough power to enable the N900 to get it charged. Therefore the advice to get a battery charged outside of the device.

What is the status of charging LED and screen output?

Possibly try to attach the charger a few times for a few minutes ...