Graphic card hook up to HDTV

I hook up my brothers computer for him and put it on all the right settings for him, but he recently went into his graphic card display setup and turned up his resolution setting from 1366 x 768 up to the next highest resolution and the HDTV went blank and said "invalid format". This has happened before and I usually just hit the Esc button and the picture would pop back to its former resolution. But my brother did it and we hit the ESC button and it doesnt go back it just says invalid format and the screen stays blank, and my brother turned the computer of from the button on the computer case, a hard reboot I guess. The computer thenn ask us if we wnat to do a safe boot and the computer will display on the HDTV on safe mode but on a normal reboot it stays blank and the HDTV screen says invalid format. Please help! WHat do I do?