Repair Programmable Thermostat Wiring

My programmable thermostat has suddenly stopped communicating with the heater. I traced the problem to what looks like a loose or disconnected wire at the thermostat base, and now the system will only sit at a default temperature—no heat kicks on at all.

I no longer have the original installation or user manual, so I need someone familiar with common programmable-stat wiring conventions (R, W, Y, G, C, etc.) who can:

• Inspect the wiring at the wall plate and, if necessary, at the furnace control board
• Reconnect or replace any damaged low-voltage wires
• Confirm proper voltage, configure the thermostat, and run a quick functional test to be sure the heater cycles on and off correctly

Please bring a multimeter and basic replacement wire or jumpers. Once everything is reattached and tested, I’d like a brief walkthrough of what you found so I know how to avoid the issue in the future.

The job is complete when the thermostat reliably calls for heat and the furnace responds without errors.