DIAGNOSIS Oils and dirt on the glass and the capacitive touch ring are interfering with the touch sensor.
FIX Power down the thermostat by pulling it off its base. Use a microfiber cloth, slightly dampened with water only. Gently wipe the entire glass face and the stainless steel ring. Dry it completely with another clean, dry microfiber cloth. Do not reattach until it's 100% dry.
IF THAT DIDN'T WORK If the ring is still unresponsive, there may be a hardware fault in the capacitive sensor. After cleaning, perform a hard reset. Press and hold the ring for 10 seconds until the screen goes blank and restarts. This reboots the touch controller.
HEADS UP Never use cleaning sprays, window cleaner, solvents, or abrasive cloths. They can damage the oleophobic coating on the glass and permanently ruin the touch sensitivity. Water only on a soft cloth.