Not much to do except a rebuild if the tubing has been breached. Once I had a mechanical temperature gauge that had not moved for years so I carefully removed the sensor, tubing and gauge from the car. Before sending it off for repair, I tried one more thing and immersed the sending unit in boiling water. The needle bounced from its position at 0* to something over 200* Reinstalled everything in reverse order of disassembly and gauge has now been working perfectly for 30+ years.
For those with similar problem, I would suggest simply removing the sensor still attached to the tubing from the engine and applying heat. You may be lucky and find that your problem is simply a stuck needle in the gauge.