DIAGNOSIS Incomplete circuit or incorrect wiring. The most common new-install mistake is not having a "C" wire or not connecting it properly.
FIX Turn off power to your HVAC system at the breaker. Double-check every wire is inserted straight into the correct terminal on the Kono's wall plate and the tiny metal clamp is gripping the bare copper, not the insulation. The "R" (power) and "C" (common) wires are absolutely essential. If your old thermostat didn't use a "C" wire, you cannot install the Kono without adding one or using the Lux Power Extender Kit.
IF THAT DIDN'T WORK Your HVAC system might not provide the correct 24V AC power. Use a multimeter to check for ~24V AC between the "R" and "C" terminals at the wall plate with the HVAC power back on. No voltage means a problem at the furnace control board or a blown fuse on the board.
HEADS UP Never assume the wire colors match standard conventions—the color at the thermostat means nothing. You must verify what each wire is connected to at the furnace control board. Label them before removing the old thermostat.