how to fix my rca -8335 dvd,record,vcr combo?

when i install a disc, it tries to read it,then says either install disc, or no disc. reading disc seems to stay displayed longer than it did before trouble started, w/more clicking. it will not read either a new disc or a recorded disc. until now it has worked great except for occasional lock up. unit is 3or4 years old, and done a lot of recorbing.