My nokia N900 fell on the floor nd it's nt working nw. It;s not being restarted..Someone please tell me wht to do???

Could be anything really of any level of severity. But first I would open it up and double check that the battery is nice and snug and didn't get jolted loose somehow from the impact (a good condition battery doesn't have enough wiggle room in the battery slot to do so, but I have one battery where I actually accidentally widened the contacts, maybe the impact caused the issue).

Personally, I would be surprised if it was a permanent problem, because I've had a N900 drop while running down stairs, whereupon it hit the cement several times on the way down, and still be fine. But you could just be unlucky and the impact may have just damaged something internally. You could always open it up and see if anything seemed loose (though like in the now infamous cases of the telephony chips and SIM card slot getting loose, the hardware can come apart enough for the electronics to fail but not be at all visually noticeable to us humans...)

I suppose a relevant question is: Did you have swap on an SD card? If the fall caused the device to disconnect from the SD card, the resultant crash could well lead to some filesystem/file corruption, although usually that shouldn't effect your ability to boot.

When you say it's not being restarted, you need to be more specific: Is it not responding at all (no LED, no screen activity, etc) when you try to boot it? Does it start to boot and then hang, turn off, or reboot itself partway through?