I just got an xbox 360 everthing work... it powers up and the sound works but there is no picture. any idea how to fix it? thanks

Clear the system cache:
1. On the console, go to My Xbox, and then select System Settings.
2. Select Memory.
3. Highlight any storage device, and then press Y on your controller.
4. Select Clear System Cache. If Clear System Cache is not available, use the following controller button sequence to clear the system cache: X, X, left bumper,right bumper, X, X Note If you don't see a system maintenance prompt on the screen, try pressing the button sequence again.

Try playing discs without the hard drive in:
1. Turn off your console.
2. Locate the hard drive on your console.
Press the release button on the hard drive and lift the hard drive from the console.
Turn on your console.
3. Try to play a game disc.

If this resolves the problem, you will need to replace the hard drive.If not, do the same thing for your memory units. If it works without them, the they may be bad.