Why am I not getting picture/sound on my dvd player?

I have a cheap technosonic 5.1 surround sound system but recently when I turn it on to the av channel there is no picture or sound when playing a disc, I have checked the scarts and they are fine. There is a built in radio and when on it shows channels are tuned in although no sound is coming through speakers. I think it could be a problem with the amp, but why am I not getting a picture on cd/dvd mode. It has nothing to do with batteries in remote, or mute setting.