No sound no picture.but when playing the x-box...

Question

I have a philips gray tv.the model number is 20pt6441/37.Here is the problem.its a cable ready tv.so all we did is put the cable that is running from out the wall right into the back of the tv.so one day there was no sound coming out of the tv,and there was no picture on the screen,but the funny thing is when my son plays his x-box there is the picture and there is sound coming out from the tv.is the tv on some typ of mode to were i can fix it with the remote.what should i do?