Missing display segments may be caused by battery issues.
1. Disconnect power from the display (and sensor, if applicable) for 15 minutes.
2. With batteries removed, press any button 20 times to clear electricity from the unit, even if the screen is blank.
3. After 15 minutes, install fresh batteries (starting with the sensor, if you have one) into the display.
4. Wait at least 15 minutes before pressing any buttons.
5. Set the time and date, then check for missing segments.