Thermostat is getting 1/10th the voltage it is supposed to?

So, our upstairs AC went out (in an old house) and the thermostat upstairs is not working. I checked the voltage supply and it's only around 2.75V, while the recommended voltage is 24V and the properly working downstairs thermostat is getting around 28V. What could have caused this, and how can I fix it?
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