Problem with Panasonic TV and Radeon HD 6450

I have a Panasonic Plasma TV. The Radeon HD 6450 is directly hooked by HDMI to that. When I have switched on both the system and TV at the very first time, there was no problem. But after the TV switched and again switched on, it gives me some trouble. Also, Windows is not able to find any audio device. I have to restart the computer to run the WMC. I am sure about that all latest drivers are installed. If anybody knows the solution for this problem, please let me know.
Thanks in advance.
