We had this on our Sky+, turned it off and on from mains but still on standby, so left it off for 5mins, then switched it back on and it worked.
Kept happening so we got a new box from sky under warranty, only the dimwits used an amstrad (or similar) reconditioned box and somehow ruined our signal. He didn't know how he'd ruined it so we said we ask sky for another box, so he refused to bother fixing it then as he'd get no commission for it now as someone else would be coming to fix the problem. Complained to sky and a different guy came and fitted a brand new Pace box. Been happy days so far.