Rebooting your device can help with connectivity issues, as well as any display issues.
For underground timers:
Unplug it from its power source for 30 seconds
Then plug the timer back into the power source
For hose timers:
Remove the battery tray from the timer for 15-20 seconds
Reinsert the battery tray, making sure to push it in all the way to ensure no moisture gets inside the body of the timer