There are certain things you need to know before you start. How many zones you have. Which number zone is not working And where the valve box(es) are.
To find these boxes, you follow the wires from the timer, outside to the valve box. This will be in the ground with a removable cover. Inside the box are a number of control valves. One for each zone you have. If you have many zones you may have more then one valve box.
The connections should be in capped connectors with a sort of grease in them. This keeps the wires from corroding.
You can manually turn on each zone from this box by turning the valve tops, one at a time until you get to the zone that's not working. Once you find the zone, check to see that the wires are, not corroded and properly connected.
This is the easiest to check. If you have a problem with this, it would be best to call in a pro to fix the problem.
Because if this is not it, you could either have a bad valve or a bad timer component.
Ron