Screen Defective ?

When I open the screen , it lights up normally , but within minutes it starts to get fuzzy , and all sorts of lines start appearing zig -zagging etc... , and progressively gets worse until the screen goes black. This occurs wether you have a dvd in it or not. if you play a dvd the same thing will happen. all other functions seem to be ok, you can hear all the audio, but no picture.

Of course this happens when my warranty has finished, what could it be , and how expensive of a repair whould I be looking at?

Please advise
Thanks
Ernie