With issues like this, you should really add a bit more info if you can.
Is it a genuine or aftermarket battery. Are you using a battery grip. Was the camera acting up before it stopped turning on. Was the charge status of the battery indicating it was bad in the battery info menu? Was the camera dropped. Was the camera used in an unsuitable environment.
The more info we get the easier it will be to help you out.
A couple of non obvious reasons why it might not turn on...
There is a switch on the bottom of the camera where the battery door is. The camera will not turn on if this is not depressed. The door depresses this when it is shut, so if the door isn't shut quite right, then it wont turn on.
Same goes for the CF card door. If it is not closed and locked into place correctly, the camera will not turn on.