Resolution change on LG HDTV

[navyseal279]

I have an emachines desktop computer, running Windows 7, with a 21" Dell monitor. I hooked my computer up to an LG 47" HDTV and it worked fine, except the resolution was off a little bit, so I changed it. Now all I see is a black screen with a small box that reads 'Invalid Format'. How can I change it back so that I can use this as my monitor again?

 

[frozenlead]

Hook your Dell monitor up and choose a lower resolution, then plug the TV back in.

If your TV displays full HD, you should set it to 1920x1080.