Portable air compressor wont turn on

1st check for power at the pressure switch. Take the cover of the switch and make sure you have line voltage at the terminals from the wall outlet.
if no voltage>replace cord or plug end with proper gauge wire.
If voltage, then, while you have the cover off of the switch **Make sure it is not plugged in** and operate the on off switch and visually check the operation of the switch. You should see some flat copper terminals that, when the switch is turned on, kinda rotate and drop onto some other copper terminals. The connection is kinda like points on a old car ignition, little pads on each copper thingie contact each other. I have seen these little pads become pitted and burnt from arcing. Try to clean these with a small file or pick if they are corroded. Get some electrical contact cleaner and clean the switch completely, *it is not only electrical, but is also mechanical*. Switches will intermittantly work, replace the whole pressure switch if there is heavy wear, charring or working intermittantly.
If this is all good, check the wires coming out of the switch for power to the motor, replace if necessary. If this is ok..
You probably have a bad motor. I have not had much success in revitalizing motors, its usually better to just replace it.