Blue screen has audio can change channels and hear different audio but no picture
If you can see menu on-screen display, just get into it, surf for blue-back option and turn it OFF first, and see whether there is any grains appears on your screen. If there is no grains, and only a plane raster, your TVs video processor Ic has got damaged, and should need replacement. If there is grains, you just retune it in autotuning option, and allow it to store all the transmitting signals at that time. This process may take two or three minutes to complete, and you should wait till then. Never forget to connect antenna socket to the antenna terminal of your tv.
If you are using set top box, and connected it to the TV through audio-video cable, your TV has no fault, but the set top boxs' video processor has got damaged. OK,