DIAGNOSIS The wireless link between the main thermostat and your remote room sensor has dropped. Yeah, this one's common. It's usually a weak battery in the sensor or wireless interference.
FIX First, replace the battery in the problematic sensor. Use a fresh, name-brand CR2032 lithium battery. Then, re-pair the sensor. On the T9, go to Menu > Sensors > Add Sensor. Press and hold the button on the back of the sensor until its LED blinks. Bring the sensor within 3 feet of the thermostat to pair, then move it back.
IF THAT DIDN'T WORK If the sensor still drops, interference is the likely culprit. Common sources are large metal objects (filing cabinets, fridges) between the sensor and thermostat, or competing 2.4GHz signals from Wi-Fi routers, cordless phones, or baby monitors. Try relocating either the sensor or the thermostat a few feet.
HEADS UP The T9 can only actively use one remote sensor per comfort profile (Home, Away, Sleep). If you have multiple sensors, make sure you've assigned the right one to the profile you're in, or it'll look like it vanished.