you want to check how many threads with same question first, and look in
Mac OS version forum that you have; and the notebook MacBook instead of the workstation forum for Mac Pro (non-laptop)
Not a lot of detail to go by with "won't boot" though.Mac OS X Help
Isolating Issues in Mac OSTroubleshooting: My computer won't turn on
Assume you know what your OS, how to use the Recovery Mode, and how to repair or do Safe boot and other tasks.
Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account
Identifying resource hogs and other tips: Using Activity Monitor to read System Memory and determine how much RAM is being used
Starting the computer in "safe mode": Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems: Apple Hardware Test
General Mac maintenance: Tips to keep your Mac in top form