Most likely the transformer has quit.
They should produce from 24 to 28 volts AC at the two screw terminals.
Turn off the electricity and use an ohmmeter to measure the primary and secondary windings.
The DC resistance should be about 30 ohms on the primary (115 V) side and the two screw terminals should measure about 3 ohms.
Of course, you should disconnect at least one side of the winding to measure the resistance.
Home Depot will have a replacement transformer.