why did my 1996 buick park ave start blowing hot air when a/c is on

Is it blowing HOT air or ambient temperture air? Thats the question. If its only blowing hot like you have the heat on, then its most likely a blend door issue. If its ambient temperture air, then likely your AC has a low charge or other malfunction. A quick check to see if your AC is working is to turn it on and listen for the click of the compressor turning on, and you should also hear the radiator fans kick on. If they do NOT turn on, you have an AC issue and I recommend recharging the AC (but you certainly have an AC leak). If they do turn on, more investigation is needed.