Sounds like whatever you hooked up didn't get along with your PC. Windows probably autochanged the res to something incompatible - and saved it (like it shouldn't do. You know, the way it always works.)
I'm going to assume it's a dual-head card. Try using the other port.
If all else fails - boot into safe mode and change your monitor defaults to duplicate on both heads & disable TV. Set your graphics to safe settings that will be displayed by anything ( low res, low refresh - or - just reset it to default. )
You should really try hooking the TV up while the PC is on - and you can mess with the config. Don't be afraid to plug in a monitor while the system's running. ( Check out that HDMI adapter. See how there's some pins that are longer than the others? Grounding - keeps it from frying. Safe to plug in while on. )