many different things can cause this. have you recently moved the television, or has it been impacted or had liquid on it recently?
1) always start with a settings reset. it doesnt sound like a settings issue, but it doesnt hurt to try.
2) Make sure your source is functioning properly. check a different source, with a different cable, on a different port, and make sure it isnt any of the things listed.
3) crack the device open, and run through the connectors to the LCD. make sure it isnt anything loose.
4) while looking for loose connectors, keep an eye out for blown components, as well as signs of corrosion. repair as necessary.
5) if nothing above fixes the issue, you are looking at one of two things needing to be replaced. The LCD, or the Logic Board. Replace the components one at a time to see which one needs to be done.