What kind of cable is connected between the TV and box?
If it's coax or yellow/red/white composite, then maybe the volume on the box was turned down or the red/white stereo cable was unplugged.
If it's HDMI, then maybe the audio output on the box was turned off or switched to a non compatible format (PCM or Dolby Digital).
Either way, it's possible the TV speakers were turned off or SAP may have been turned on (and there isn't an SAP channel).
Most of this is accessed through a series of menus on the TV or box... other than the box volume which might be accessed through the front box controls or remote if it's in the right mode and the volume controls are unlocked.
Then again, none of this may have happened and the box or TV went out.--
Two is one, one is none. If it's important, back it up... Somethimes 99.999% availability isn't even good enough.