Motorguide W55 was working fine and then suddenly stopped. It turns left and right but the propeller will not turn   anymore.

Does it make noise when you hit the on button? It’s either weak battery/connections or maybe the prop pin broke. You will hear it spinning inside still if the pin broke 

Pin is easy to check, pull prop off and look for damage. Hook power straight to troller cables with 12V and check. Check wires for any visible damage if it didn't have a resettable circuit breaker. 

Fishing line gets wound around the prop shaft and should periodically checked so the line does damage the seals. 

There is a shear pin to protect the prop from breaking and is simple to replace.

Power could be a tripped breaker or fuse or bad on-off switch or wire connection. Worst case is the motor needs new brushes and or rebuilt if water got in.

Tom