First reset the SMC: http://support.apple.com/kb/HT3964?viewlocale=en_US
Boot from your original OS X Installer Disc One that came with your computer. After the chime press and hold down the "D" key until the diagnostic screen appears. Run the extended tests for a minimum of two or three hours. If any error messages appear note them down as you will need to report them to the service tech when you take the computer in for repair.
Some "common" error indicators:
SNS - sensor error
MEM - memory error
HDD - hard disk drive error
MOT - fan error
If you cannot get the diagnostics to run and neither of the two above resets help, then you've had a hardware failure. Let us know and we can proceed.