Welcome to the forum
Has anything changed - new cables, you've moved the unit or anything, before this started happening, or is it just all of a sudden?
Have you checked the Troubleshooting section of the manual?
As a last resort, restoring the factory settings might do the trick - you will lose all your stored channels though. If you should need to do this, it should tell you how in the Troubleshooting section of the manual.