A/C suddenly stopped working - W126

[86560SEL]

Yesterday I was driving mothers W126 (she had picked me up, one of my cars was at the shop). The A/C was working (not ice cold, but was pretty good) when we first left.. stopped at the bank and made a deposit (shut off the A/C and car while in line, it was moving very slowly), but when we took off again, the A/C was not cooling. Blower is still fine, but only putting out heat. The compressor is not kicking on - I cant hear it kicking on anyway. Could the refrigerant have leaked out that quickly, or the compressor locked up, or? Just trying to figure out what the most likely cause would be. Thanks.
btw, its a '88 300SEL.

[S-Class Guru]

Try a couple of things before panicing.
With the engine off, reach down and try to turn the compressor. With the clutch disengaged, it should easily turn over by hand - and listen/feel for anything rough or binding.
Find the drier/accumulator and find the two sensors that are screwed into it.
The low-pressure switch has two spade plugs directly on it (the fan switch is different, it has two 4" wires attached directly to it, and the spade plugs on the end of those). Unplug the two low-pressure switch wires, crank the car, start the AC, and short the two wires you removed from the low-pressure switch together with a paperclip or something. The compressor should run.
If it does, check the sight glass on the top of the drier for bubbles or evidence of freon flowing. If all is okay and sounds fine, let it run a short while to see if the low-pressure hose gets cool. If it does, it may be a bad pressure switch - or a lot of other things also. next step is to check pressures with a gauge set.

If the compressor does not run with the swich wires shorted, it may be a fuse, or the mAS relay controller (or you may have a separate relay on your '86, I can't remember. But check the fuse compartment.

Try these and get back to us.

[86560SEL]

Well, I was going to check it... but I tried it again today and it works fine (but not as cold as it was). I am guessing its low on the gas?

Dont matter right now anyway because its setting in the parking lot at a supermarket and will not start.