I cannot tell you exactly what you will need to replace. You did not state if the motor is making any kind of noise or not. If the motor is not making noise you will have to verify the outlet has 120 volts. Then remove the cover of the pressure switch and check the connections to the switch then verify that the switch has 120 volt going out to the motor. If the motor has 120 volt going to it you will need to replace the motor.
Working with live electrical current can be very hazardous and possibly deadly. Testing voltage output requires working with electrical current. Do not attempt this measurement without the proper tools and safeguards.
If the piston is causing the motor not to move you will need to remove the covers around the compressor head. Remove the four bolts holding the head on. Now inspect the valve plate and the cylinder. You will need to replace the parts that are worn. I have included an image of the compressor parts list below.
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services
Here is a link to buy the parts needed from our parts web site
Sears Parts Direct