There is more information necessary to begin troubleshooting such a problem, such as: type and model of clock, is this zone stuck on or simply is the only one that will run when the clock is functioning, have you reset the controller, were the other two zones functional and suddenly ceased working, any additional information you can provide will help narrow your problem down.

Thanks for your reply. It seems to be stuck on one zone. When I noticed dry patchy areas in the other two zones, I turned the system on manually, but when I turned the switch off and then back on again, it didn't advance to the next zone.