If the inner basket does not rotate at all then that will shutdown the machine. You did not mention which model you have. If it is a digital unit then you should get an LR error code which is locked rotor.
There is a diagnostic sheet in the top console section that has a wiring diagram and diagnostic help. Locate that sheet and perform the motor drive test. I would also check the connectors on all of the circuit boards. It is a good idea to pull each connector and put them back on to clean the connectors as they can oxidize over time and make bad contact. See
Here for additional motor drive information.
If you have the older motor drive then you will need the motor conversion kit which has a motor, motor control board and wiring harness. If you have a newer motor drive you will only need to replace the motor control board.
If you have a timer knob based Neptune, don't forget to replace the original brown tipped door latch wax motor. See
Neptune Help Song for a video that explains what fails. All of the stand alone timer based Neptune's shipped with this faulty wax motor that will eventually fail and damage the R11 resistor and Q6 triac on the machine control board. It is better to prevent the failure so you don't risk damaging the machine control board.
The digital Neptune's do not have a wax motor but a solenoid that locks the door.
Another thing that will shutdown the Neptune's when filling is the water inlet screens. These can be checked by removing the water hoses and looking up into the water inlets with a flashlight. Use tweezers or needle nose pliers to pull out any debris in those screens. If the screens are clogged it reduces the water flow and it takes too long to fill which will shut down the machine so it does not overfill.
I hope this helps...
Jeff Hartman