This is a common problem and there is an easy answer - mine did the same whilst on holiday and I phoned Thetford Technical Department. What happens is that a small circuit board under the silver grey push-button panel on top of the loo drops slightly out of place because you push too hard. You take a fine knife blade like that on a stanley knife or something and put it under the edge of the silver grey plate and this plate peels off and remains sticky for when you put it back. Then take a small electrical screwdriver or the small point of scissors and put it in the samll recess of the plastic plate (at front mid-point) beneath the silver plate and prise this plate out. On the underside you will see a small printed circuit card that has slightly dislodged. It locates on two small plastic pegs and is held in by two platic clips that are not very strong. Just push the circuit board back in place firmly, then repace the plastic plate in the recess, then carefully stick the silver grey cover back on top and that should solve the problem.
Regards
Alan