I have a similar problem
I went away for few days last week. Returned to find video had taped stuff I timed on the day I went, but the Sunday stuff was a blank soundless screen. Set video to tape stuff Monday - OK. Set it to tape stuff yesterday - blank soundless screen. Set it this morning to tape stuff - got picture but no sound.
These faults are rectified by simply going up a channel and back.
Just phoned CS - "Call us next time it happens".
Anyone out there any ideas?