If the refrigerant level is too low, the Low Pressure Cut Out - LPCO - switch will disable the compressor clutch to prevent two things: 1)freeze up of the evaporator and 2)insufficient lubrication of the compressor. You can jumper the switch, most are mounted on the accumulator or the suction line leading to the compressor from the evaporator, and see if the compressor engages. Do NOT leave the jumper in place or you will cost yourself a pile of money. If it engages, you need to determine if the switch is bad or the refrigerant level is too low for safe operation.
If it doesn't engage, try thwacking the clutch with the engine running and A/C demand present to see if the clutch gap may be too wide. Be careful, as I did not tell you to do this...
tom