When I've seen this happen, it's usually due to a couple things:
1. The coaxial cable is bad.
> Make sure the ends of the cable are screwed on tight.
> If one end of the cable goes to an external antenna AND it's not 'booted / covered / weather o-ring sealed' then water or moisture may have gotten into it and needs replacing.
> Without a cable tester, the quick and dirty is just to replace the cable with a new one to rule out the cable.
2. The connection and/or tuner on the TV has gone bad.
> I've seen this happen after power surges. If your TV is not on a small power strip with surge protection, then it's likely taken a power surge which may have messed up some components. I'd need the model, etc. for more troubleshooting. But in these cases, outside of manufacture warranty, smaller TV's aren't worth the repair. A larger screen (> 40") is usually where it's worth the repair -but I'd still price parts, etc. to see.
Another possibility is the antenna -it could have taken a hit or just gone bad (depending on what kind you've got).