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Hello, the thermostat most likely needs to be replaced.
However, you can try removing the batteries, waiting 5 minutes, then replace the batteries backwards, wait 5 minutes with the thermostat turned on, then reinstall the batteries in the correct positions.
That has about a 20% chance of solving the problem.
We can go from there.
Hello again, it should have standard AA batteries in it. Let me know if that is what was in it or not.
The old style mercury bulb thermostats will last indefinitely.. 50 years is not uncommon... it is just that these were replaced by *electronic *programmable thermostats, those generally go 10 years or longer.
The Pro Tech brand has not seen much distribution at all for some reason, perhaps due to a higher failure rate, but I do not know why for sure.
If you can remove the front of the thermostat and access the thermostat wire connections and tell me which color wire is attached to each terminal I can tell you what thermostat replaces it.
I also need to know if you have a heat pump system, that runs the outside unit in the winter as well as the summer, or a more conventional system, electric cooling unit outside and a gas furnace inside.
Let me know, we can go from there.