Do you happen to have two outputs, such as the TV along with another monitor, connected at the same time? If so, remove the second output and leave only the TV. Also, you can try booting up with the computer connected to a working monitor first and plug in the HDTV when you reach the desktop, leaving the first monitor still connected. Then, go to display properties / screen resolution to check if the computer detects two displays. Activate the second display (TV) and set it as your main display.