If the oil has drained out of it , the air con system will not engage the electronic clutch , this is to prevent damage to the compressor / system.
The Ec light will stay on indicating that the system is keeping it disabled ... no need for you to switch it off yourself.
I've got this on mine at the moment , a very common problem with CLK's is that the condensers rot through. ( which is what is wrong with mine )
Incidentally, when you do get it fixed , never turn the air con off ( a lot of people turn it off for the winter , and find when summer comes that all the seals have hardened where no gas , which is also the lubricant has flowed round the system and the gas has leaked out and they have to have new seals ) ... if you run it during the winter as well , it actually dries the air taken in and will make your windscreen demist a lot quicker