Most likely your system is low on freon. Since the compressor's oil is mixed into the system with the freon, the low pressure switch won't let the compressor come on if the system is low or empty. Running the compressor without oil will kill it quickly. So your system will need serviced/recharged.

It's also possible that the low-pressure switch is bad, and is saying you don't have enough refrigerant/oil in the system even though it's fully charged. The easiest way to diagnose this is to check the charge in the system. If it's low, you've found the problem (charge it). If it's not, you've still more than likely found the problem (pressure switch).

To check the clutch, on my Toy there's a connector in the clutches wires about 6 inches from the compressor. Most liekly this is where you found it's not getting power. You can hook it up straight to the battery for a second and listen for a loud CLICK. Which wire is + and which is - doesn't matter, the clutch is just an electromagnet, and I've never seen one that was polarity-sensitive.