Bill,
Normally with the C250 it is normally the pump that gives problems having bad contacts from the PCB under the push membrane, but yours seems okay If you have water in the tank have you looked for a leak in the pipework caused by frost damage or disjointed pipe? Certainly there is no problem with the electrics. The other cause could be the black jelly like gunge that builds up when using some flush fluids. Assuming no water leaks then I would drain out the flush tank. Refill with clean warm water a couple of times and drain. Then using warm water and some bicarb refill and use the pump, then leave for a couple of hours. It should start to dissolve the black gunge then you should be able to start flushing it through.
Keen to hear how you get on.