Did this happen yesterday or today? They shut off analog service yesterday if you've been living in a cave for the past 2 years. Perhaps you don't have a digital signal coming from your cable. If you've piped in from a neighbor's signal, that might be the problem. If you're actually paying for the cable, call your provider and give them a "WTF?"
If this didn't start just yesterday, then the TV may be on its way out or you've got a frayed coax cable somewhere in the house that may gotten bumped loose or bent at an extreme angle which can drastically reduce picture quality.