Need help. My N900 goes off after a couple of seconds. I thought it was just a power save mode but when I found the power save mode and uncheck it, the phone keeps on shutting down and i can't receive calls. Those calls goes straight to voice mail. Also when I lock the screen and keys phone goes off and can't receive any calls it just goes straight to voicemail. What to do with this?

Any third party software installed?
Reboots are currently an issue being worked on, see bug 6334 and bug 6350 in bugs.maemo.org that are related to it (please read the comments and avoid adding a "me too" comment because it creates mail for everybody).
Also this might not help at all, but did you try reflashing?

I don't know how to reflash. I'll try taking the battery off for a while.

I had the same problem. After i bougth my n900 and start the phone. After a couple min it went off and restart again and so on and on. Until it hang in the begining on loading screen with the 6 dots.

Next day i went back to the shop trade in my n900 for a new one and this one works perfect.