I would start with the poly-v belt. If it slips even slightly, it may cause the a/c clutch to lock out. I don't know if it's supposed to reset automatically after this, but it has been my experience that the a/c will usually not work until you turn the engine off and restart it. If you have a code reader, you may see a DTC code 7 (poly-v belt slips) on the base module (socket 8). If you haven't changed the belt in a while, it may be a cheap fix.

Also, if you are in a really hot climate (like here in Arizona), the a/c pressure may exceed the limit and prevent the clutch from engaging. This has happened to me a few times in the hot summer where I have parked the car with ambient temperatures of 113 F and the under-hood temperature must have been incredibly high.

Also, I was told by a tech once that under really high temperatures, the pressure in the system can be so high--even with the engine off--that when you go to start the engine and engage the compressor it takes a lot of force just to initially get the compressor turning and it could cause the belt to slip and lock out.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles