99 Pontiac Firebird ac not working?

My 99 Pontiac Firebirds A/C completely stopped blowing ANY air thru the main vents. It still blows very cold thru vents on top of dash (the defrost vents) and the foot area. I took it to Pep boys for another problem but they lookeda t it and brought me back a small black disk (kind of like a small hockey puck) with 2 prongs on the end (the whole thing made of plastic) and said "thats your problem." I asked them to put it back on until I have time to get it fixed. Now I have a mechanic that can fix it but cant rememeber the name of it. I think he said the part would add "about 5 bucks" to my bill. Any suggestions? Thanks Everybody!!