Ive had that happen with mine before where a clog prevented it from doing anything. If i remember correctly it happened after we were anchored out for a weekend and the batteries were low so the motor didnt have enough power to grind it up properly and it locked. Try bringing a little plunger with you and plunging while turning the switch
Btw Since that incedent if the batteries are low Ill start the start the engine to flush and proudly declare to my wife that it takes a small block chevy to flush when Im done
almost 4 bills later, but at least it was delivered in less than 48 hours (on a Sunday no less) and ready to put in....