This is the most common irrigation call I get in a Sacramento summer. One zone will not come on, the rest are fine.
Nearly always it is one of three things, and you can check two of them without me.
One — the timer. Sounds obvious. It is astonishing how often a zone has been switched off at the controller, or the run time has been set to zero, or the whole thing lost its programming after a power cut and defaulted to something odd. Look at the controller first.
Two — the valve. Each zone has one, usually in a green box in the ground. If the timer is calling for that zone and nothing happens, the valve solenoid may have failed, or there is grit in the diaphragm holding it open or shut. That is a repair, not a replacement, most of the time.
Three — the wire. A single dead zone with a healthy valve is very often a broken wire between the controller and that valve. Landscaping work, a shovel, a gopher. It happens.
If it is heads spraying sideways, misting, or one dry patch in an otherwise green lawn, that is a different conversation — usually heads, sometimes pressure.
Either way, do not let a dead zone run all summer. A dry patch in July costs more to bring back than the repair does.