The N900 needs to be "on" to charge. this means a completely dead battery will not charge in the device.
"on" doesn't necessarily mean booted up BTW, there is a state called "ACT_DEAD" in which, the device can charge but the device is "off" not booted.

If you get a solid yellow this normally means the battery is very low but can be charged, you should leave the device to either boot up on it's own or pulse yellow before booting up the device.

It will also not switch on with no battery.

I would consider a good quality new battery if you haven't replaced it in a while, Li-ion batteries don't last forever. Especially, if you run them completely flat.