How can I reset the ac unit. The ac off light is on and it blows hot air.

I have had the same problem twice over the last five years in my 2003 e500 sport. We could not find any reset protocol and finally took to a Mercedes mechanic who reset with computer somehow. I have unlimited access to alldata and was unable to find any assistance there on the internet. I think the problem may be related to the automatic shut off for the compressor when Freon low and then the failure of your aux battery seems to not allow for the resetting. Freon will be full and the compressor will not work without resetting but no ability that I am aware of without taking to shop