guessing the battery is flat?

try hooking it up to a wall charger ( the original if possible ), there should be an orange light come on in the bottom left corner, leave it to charge for a while

the n900 from memory supports an older "dumb charger" standard so you may need to try a few different ones if you do not have the original charger

steady orange - recovery charging, the battery is to flat to "boot into normal charging mode"
blinking orange - normal charging
steady green - charged