The irrigation man can test the voltage at the output of the timer for the
zone in question, to see if the timer/transformer is putting out voltage to
that zone. If it does show voltage, then he will go out and check for
voltage at the END of the wire for that zone (where it attaches to the
valve.) If no voltage, then of course it is a wiring problem. If not,
it is a valve problem, or a valve solenoid.
The suggestion to switch the wire from a known-working valve is also a good
method. If the switched wire (from a zone now working ok) powers up the
valve, then you know you don't have a valve problem, and you concentrate on
the wiring or the timer/transformer.