It could be that the video output has blown. Sounds and everything was there for me but no picture either. Contact xbox and if you pay $75 i think, then they send you another xbox withing 3 days of you returning your broken xbox.
Need to re-flow the HANA and/or GPU. Your best bet is to have a professional repair this for you. Any heatgun repairs, penny-tricks, overheating,etc, are all temporary that cause more problems than solutions. They all hurt the motherboard in ways that make a professional repair harder and less likely to last. [member=icy-tank] is the only person on Se7enSins I would trust my console with. He is the most successful, most trustworthy, and most reliable repair man here. Here is a link to his shop.