Since you had work done under the hood, I am still thinking that the tech readjusted your a/c belt a little too loose. The belt need to be very tight but not too tight that causes damage. What test did you do to make you think it is locked up? If you hear very very loud noise and see smoke coming up, you have a locked up compressor. Otherwise, get a second or third opinion before spending the big money. A defective belt can make noise too.