Are you saying that the TV doesn't recognize the computer plugged into the HDMI port, or the computer doesn't recognize that another display has been connected to the computer?
I've forgotten what the Display Settings menu looks like in Windows 7, but you should be able to right click the desktop and select settings or some such option. See if the TV is shown but grayed out. You might have to specifically enable it in the display settings.
My next suggestion is to try a different HDMI cable. And of course if you can try the computer on a different TV to ensure the HDMI port on the computer is still working.