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Most common issue is low freon. Even if it is low on freon it can still have pressure:
Now if you are certain it is not low on freon, then it is electrical. The first test to do would be to apply power directly to the compressor to see if the clutch pulls in. If it does not: Replace the compressor/Clutch (most come together).
If it does pull in then follow these steps to diagnose the issue
STEP by STEP Air Conditioning Troubleshooting