My mercedes benz 400 sel airconditioner is only blowing hot air what could be the problem

You are out of freon or have a leak that caused you to run out.If your a/c system has never been fully changed over,it has to be done by a dealer,new freon or fix leak that is.Pre-1995 cars have the R-12 freon system and shops have to have a special license just to buy,sell,use,dispose of R-12.1995 and after have R-134a freon in them.The 2 systems are not compatible with each other at all.I have heard the R-12 is real expensive now but I do not know how much.Call a couple of shops and tell them your situation and see what they say or recommend.I would call AAA approved shops because they have better standards just to have that approval and give 10% discount on labor if you have AAA.
Good luck! Greg

Low freon is most likely issue, but dont forget to look for leaks or you will never be able to keep pressure in the system.