The AC quit working in my car. What could be the problem? on 2005 Chevrolet Malibu

Wife took the car to have an oil change 2 days ago. Auto shop also replaced the coolant. I need a list of possible symptoms to look for so I can diagnose the problem.

Asked by for the 2005 Chevrolet Malibu
ck for a/c compressor unplugged,or cooling fan operation or coolant level to start with