Check connecting wires to and from any source AV equipment, that may be loose/disconnected, to include A/C power. Yes, even sitting idle, cables may work themselves loose at times as odd as it sounds. Plus it's the next easiest thing to check besides what bnewlands32 suggested.
If still no-go. Unplug/replug your AV gear in and let it all reboot. Cable and DVR boxes sometimes do this "lock-up" for lack of a better term and require a a hard-boot by unplugging/replugging.