If the pump is operating then it could be an airlock in the system. I've not seen the c2 but if you know how to access the pump remove it and plung it up and down in the water tank. This works with the external fresh water pumps. Several times mine's airlocked and when plunged up and down quickly several times seems to purge it. Try it! sorry can't be any more help. Mind you the other thing is the feed pipe to the toilet bowl may have come adrift from the pump or toilet bowl flush tube, so the pump is just pumping back in to the water tank.
Bob, May 2007