Should be a limit switch with a red button
sticking out. push it and that should reset the
thing. It could also be a bad board. Would have
to replace that and that should fix. If not then
just get another one.
Tighten power supply terminal and have circuit breaker tested. Have the compressor checked for faulty or worn bearings. Have compressor checked for worn motor winding.
I would suggest first of all referring back to your owners manual if it is available. If not try either looking in the yellow pages for a professional or even try looking up compressors on the internet. There are probably hundreds of sites that could help you there.
If you need to reset the entire system, look on the back of the unit or under the filter door (depending on the model) for a red or black reset button. You will need to press until you hear a click and then restart the unit.