First check the power at the compressor, put a volt meter or test light on the two wires on compressor, leave compressor hooked up and engine idling, it will have 12volts(13.5v) when compressor should be on.
If the voltage is cycling on and off then there is a problem with the pressure in the system.
If voltage stays on but compressor cycles off the field coil or air gap on compressor is the problem.
If you disconnected the lines and didn't plug them, then there is probably moisture in the system, and that can cause a plug.
You may need to vacuum out the system and replace the drier as it has absorbed too much moisture.
The pressure switches detect high pressure after gasses are compressed, this prevents lines from exploding, and low pressure switch when refrigerant is low, this prevents compressor running dry and burning up.
The gases are compressed which heats them up, this is the high pressure side, they are then allowed to decompress in the cab which causes the lines to cool rapidly, even freeze on the outside, this is the low pressure side.