DIAGNOSIS The thermostat has lost power. This is almost always a wiring or power-sharing issue, not a dead thermostat.
FIX First, check your furnace/air handler's power switch and breaker—flip them off and back on. If that doesn't work, the "C" wire is likely not providing a complete circuit. At the thermostat, pull the display off its wall plate. Check that the small connector from the plate to the display is fully seated. On the wall plate, ensure the wire labeled "C" is securely inserted and not loose.
IF THAT DIDN'T WORK You may have a system that can't provide enough constant power. Lux sells a Kono-specific Power Extender Kit (PEK) that wires at your furnace to create a "C" wire substitute. This is the official fix for incompatible HVAC systems.
HEADS UP Never just connect a spare wire to "C" at the thermostat without verifying it's connected at the furnace end—you could cause a short. If you're not comfortable with low-voltage wiring, this is the point to call an HVAC tech.