In the event only one zone has failed, determine the cause with the following:
1) rescan connected zones
1b) if the zone does not appear, the issue is likely a disconnected wire, else:
2) Swap the wires from a working zone with the non-responsive zone and run a test.  
3) If the zone swap resulted in the other previously working zone becoming non-responsive, then the board may be at fault.
3a) move the wire to another available zone connector.
4) if the non-responding zone is still not working.
4a) go to the sprinkler valve and verify the wiring in connected properly.
4b) if the wiring is connected and continues to not function, an electronic solenoid may be at fault.