Sounds like the motor may be seized.
Remove power from the unit and try rotating the impeller manually.
You should be able to do this by rotating the motors armature fan so you do not have to dismantle the pump assembly.
If you did not add water to the sump after replacing the impeller and before running the unit for the first time then you may have overheated the motor seals seizing the motor.
These pumps do not like to run when dry.
Melted plastic in the solenoid is often caused by a timer problem re: power to the solenoid staying on for too long.
Power to it should be momentary, just long enough to pull the flapper valve down then the water pressure holds the flapper in this position until the drain cycle removes the water pressure.