Temperature gauges sometimes "stick to the peg" on these cars. My 88 did it occasionally, usually when it was cold out. If it didn't come up by itself when the engine was fully warm, a thump on top of the dash (careful not to crack the brittle plastic!) would return it to normal operation. This is caused by either a poor connection on the back of the instrument cluster and/or old oil on the pivot of the gauge needle, that has become thick and gooey after 20 years (and is even more so when the ambient temperature is low) Nothing to be greatly concerned about, and very likely just a coincidence that it happened while the nonfunctioning AC was on.