My HDTV that I have been using fpor a while with no problems now says "Not Supported Format" after my gf changed the resolution SETTINGS. I understand that I must go back in and change whatever she changed back but how do I get my screen back when i can't see anything???I don't know much about this, but I can think of one thing.

You may have to go back to original factory settings (DEFAULT). Sometimes in the manual it will show you how to reset and go back to default. Often that is done by holding two buttons at the same time (on the TV or the remote) for some given number of seconds.

Sometimes the manufacture does not put this in the manual but will put it on their website. Or you may have to call them.

Hope this helps,
Harry

EDIT:
I have reread your post and now realize you may mean that settings were changed on your COMPUTER and not the TV. In that case, resetting the TV to factory defaults may not help.Well, SINCE its most LIKELY the computers resolution has been changed and i don't know what OS  your using here is how to change it for XP         http://www.microsoft.com/windowsxp/using/setup/personalize/resolution.mspx

and here is how to do it on vista/7

http://windows.microsoft.com/en-US/windows-vista/Change-screen-resolution

Hopefully this will help