0. Check the outlet it is connected to. (this has gotten me a couple of time with GFI outlets. :-)) If the GFI kicked the last time the unit was used then just reset it and go on with life. If the outlet is live (test with lamp or test gear) then on to 1.
NOTE: One of the most common issues is the start capacitor on the side of the pump motor. If the unit is connected to a power source and the pressure gauge is reading 0 when turned on and does not start at all, this may well be the issue. Otherwise it could by be the pressure switch cutoff is stuck. Locate and lube with VERY thin oil. I like Lock ease it is a petroleum distillate with a suspended graphite power. (DO NOT GET IN CONTACT POINT AREA ONLY IN MECHANICALS TO THE SWITCH.)
1. disconnect unit from power. (I like to hang the plug where I can always see it while working on the unit. If I don't see it when I return to work on the unit I can safely unplug it and go about the repairs if some less thoughtful person plugged it in while I was out.
We will not check the linkage between the pump and the motor to make sure nothing is jammed. With power DISCONNECTED turn the motor pulley to ensure freedom of movement. If this beastie rotates then off to the next step.
2. Check the pressure gauge to ensure that the tank is not actually already full. (Hey I have a Ph.D. and I actually have had this happen early before coffee.) Good hearted ribbing from Lawrence helped me to avoid that one in the years that followed. :-)
3. If the pressure gauge is reading above 50PSI then adjust the output regulator to full open to deliver pressure to the output line. Connect an output line, and a nozzle or other pnu tool to release pressure from the tank. and test for start by fumpping off pressure to 0 at the gauge. Then connect the unit to a power source and turn to the on position.
If this did the trick make sure to dump to pressure after turning the unit off. Take the unit outside and set it on the grass (while wearing eye protection open the petcock on the bottom of the tank. Move the unit around to drain water out of the tank. )
If this did not do the trick then you need to check the start capacitor on the side of the motor, the pressure cutoff switch and the wiring to the motor. Finally the brushes in the motor may have worn to the point that there is no contact at the commutator of the pump motor. The brushes are a pair of carbon blocks that rest against the commutator to supply electrical power to the coils within the motor which results in repelling and attracting magnetic fields and thus rotary motion. You can pickup the replacement brushes at a sears repair center or a very well stocked hardware store. (I just LOVE those places)
Let me know if this got you squared away. If not, then supply me with the results of these tests and I'll set you a hopefully more fruitful path.
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/john_28069c9efb56199d