Sound but no picture. I was wondering if you could give me some ideas to check?? I am writing on

Question

I was wondering if you could give me some ideas to check?? I am writing on behalf of my father I set up his bravia 5.1 home theater system. Everything was working fine, they were able to watch movies t.v. etc and surround works fine. He just called me and said that they are trying to watch a movie and they put the dvd in and they have sound but no picture. I am unable to fix the problem seeing I live 200 miles away from him. I am sure they just hit a wrong button or something. Any Ideas how to fix this..Thanks in advance