It will be one of two things. A bad wire connection or broken wire, or the solenoid has failed. A bad wire connection is the most likely.

Thanks for the tip. Do you know the best way to determine which solenoid belongs to a given zone? My system has two in-ground boxes with solenoids, but there's no numbering system that identifies which is which. I slightly rotated one manually because it appeared to be a bit out of line with the others. Unfortunately, this backfired as it activated a zone with a sprinkler head aimed right at me! I like to find solutions on my own, but I'm ready to call the pros....Ambrose

sorry to say this but the simplest way to find which valve does which station is to turn each one on manually at the valve. unfortunately this means sometimes getting wet.