A thermostat that shows a blank screen, won't power on, or won't trigger heating or cooling has a short list of likely causes.
Dead batteries is the most common and the easiest to rule out. Even thermostats with C-wire power often still have backup batteries; if the C-wire power is interrupted, the backup batteries run the screen until they die. Swap to fresh batteries and check again.
A tripped float switch on the evaporator drain pan is the next most common in Sacramento homes with AC. The float switch cuts thermostat power when the condensate pan fills with water — that's by design, to prevent overflow. Check the drain pan at the air handler, clear the drain line, reset the switch, and the thermostat comes back to life. Recurring trips mean the drain line needs a thorough cleaning and possibly a drain-pan tablet.
A blown fuse on the HVAC control board is the third. A low-voltage short — a nicked thermostat wire, a stuck contactor, a damaged transformer — can blow the 3A or 5A fuse on the air-handler's control board. Thermostat goes dead until the fuse is replaced and the underlying short is found.
A genuinely failed thermostat is the less common diagnosis, not the first. Thermostats from reputable brands routinely last 10 to 20 years. If every other cause has been ruled out, thermostat replacement is a $250 to $500 service — and a good opportunity to upgrade to a smart thermostat if the existing wiring supports it.