Jan, you have to start with the basics, and verify if the compressor is turning when you engage the system. This is a visual check by removing the bulkhead cover or observing the front of the compressor from some other vantage point.
The belt will always be turning the pulley regardless of how the system is performing, what you are looking for is the front compressor plate / clutch. If the clutch is stationary, then something has disabled the system. If the clutch is turning, then you have probably lost refrigerant or the condenser fan is not turning.
If the clutch is not turning, then you might have also lost refrigerant and the low pressure switch prevents the compressor from engaging. From here, you need a pressure test of the system.
The most likely diagnosis is you have lost refrigerant from a leak.
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