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Hi and welcome to Just Answer.com 9 times out of 10 this issue is related to a hardware issue usually relating to the Logic board but there are a few things we can try to make it work just to rule them out Do you have the OS disc for your computer? One thing you can try is to boot the computer up with the disk in the drive and hold down C this should have your mac boot to your disk
If the OS disk wont boot the next thing we can try is resetting the CUA/PMU The steps from apple can be found here http://support.apple.com/kb/ht1939 Basically
Let me know if either of the steps provided here help with the issue Also while you have you computer open for the PMU reset you may want to look at the memory and video card to make sure nothing has come loose.