Start with the display and manual
Some thermostats use batteries, some use system power and others use both. Check the exact model’s user instructions. If it has an owner-accessible battery compartment and a low-battery indication, follow the specified replacement procedure. Do not assume every blank display needs batteries, and do not remove wiring to investigate a loss of power.
Review the settings you can see
If the screen works, check the chosen mode, requested temperature and any schedule or hold. Note whether the display responds normally but the equipment does not. Tell the technician about a recent power interruption or thermostat change. Avoid a factory reset unless instructed for that exact model, because configuration may need to match the heating and cooling equipment.
Do not jump thermostat wires
Connecting terminals together or guessing from wire colour can damage equipment or defeat normal controls. Leave wiring and equipment diagnosis to a qualified technician. If you are considering a replacement, bring both the thermostat and HVAC model numbers so compatibility can be assessed. Comfort Nest’s Shop includes simple and programmable options; choose based on system requirements and the features you actually want.