My computer display suddenly wont work on my tv?

I just bought a hp compaq and after some fiddling with the resolution setting it was working fine with my samsung plasma... until a few days ago when the edges just suddenly went outside the edges of my tv. I have gone from 5 or 6 resolution settings to just 2... wtf happened? i think my mate who is staying with me has done something but he is so computer illiterate he just clicks on stuff and cant tell me what he has done... please help it really annoying (win7 btw)