A typical pressure gauge is activated by a bourdon tube. The temperature gauge is a pressure gauge operated by the bulb with a gas inside the pushes on the tube inside the gauge itself. There's a linkage inside that probably has a gear involved and it may have slipped a tooth or three. You might be able to fix it, but you're probably better off finding a replacement.
Here's a video that show how a typical pressure gauge works.
www.youtube.com/watch?v=9avIVZP1Ka4 The first part of the video is probably similar to your temperature gauge.
