Suggests either the box or the receiver have defaulted back to settings incompatible with each other, but which the TV can handle. I'd check things like video output format, resolution, TV standard etc...
Or it could be an handshaking problem, in which case the solution may be to turn everything off, disconnect TV, receiver and Virgin box from their HDMI cables, turn them all on again, then turn off, reconnect, and turn on again.
Yes, I know it sounds crackers, but then HDMI handshaking is a riddle wrapped in an enigma, etc...