Your video card is outputting a TV standard that your TV can't understand.
Typically, this problem is caused by displaying an NTSC (US standard) signal
on a PAL (UK, European) TV or vice-versa.
Go to your display settings and there should be an option in there to change
the TV output standard. On certain older video cards (mainly ATI) it's done
by changing a physical jumper on the card itself.