Hi. I'm Steve. I'm sorry to hear about your TV. I will try my best to give you a precise and accurate answer.
The TV is faulty. If you still have audio, I would suspect the YSus or YBuffer boards. To track it down more precisely would require you to remove the back from the set and start troubleshooting with a multimeter. I can help there.
Can you do that?
If not, then you're facing a bill in the $300 range and up. keep your eye on the cost of new 50" plasma TVs - about $500.
Let me know how you want to proceed.