As Steve U has mentioned it is possible that you moved the A-b-c switch. In that case we would recommend performing a hard reset with high quality alkaline batteries and making sure that the A-b-c switch inside of the battery compartment of the display and the outdoor sensor are on the same letter. To perform the hard reset please follow the instructions below.
1.Bring both the sensor and display unit indoors side by side. 2.Remove batteries from the sensor. 3.Remove batteries from the display unit and unplug the AC adapter (if applicable). 4.Wait 1 minute to ensure any remaining battery power is discharged from both units. 5.Change the A-B-C Channel on both the display and sensor making sure they both match. 6.Reinstall batteries in outdoor sensor first. 7.Reinstall batteries in display unit second. 8.Let the units sit within a couple feet of each other for 20 minutes to ensure a wireless signal is received. * The display should receive the signal within several minutes. 9.After a connection has been established you may place the sensor outdoors.