ac in my 2008 MERCEDES ML350 blows hot air while i want cold air

What is your question?

today the ac temperature will be so cold that i have to turn the button down to the lowest level and tomorrow, the ac will just blow hot air. took the vehicle to the mechanic who did not detect using the computer where the problem is but, suspect it could be temperature sensor or compressor. does any body has similar problem and how did he manage to solve that?