There is only a temporary fix that doesnt always work called "The Towel Trick". Now this is going to sound like I am shitting you, but I really am not. Take off the hard drive, disconnect the video cable, and leave the power connected. Wrap the 360 in several towels. Turn the system on, and let it cook for half an hour. Reattach everything and see if it worked.
Again, this is only a temporary fix, the system has to be sent in for proper repair.