Left it unplugged overnight and it seems to have come back.
I noticed before the sound disappeared that it was doing something weird with the volume control. I turned the volume on the box all the way down whilst I was on the EPG, but then when I selected a channel, it told me it was at full volume, preventing me from turning it up - even though it was turned as far down as it would go.
I'll just need to avoid using the volume control on the box and stick to the TV in case it happens again.