Turn off the water supply. Open the bonnet of the sprinkler valve and unscrew the solenoid, taking care that you don't drop the plunger and the spring.
Open the solenoid housing and inspect the plunger. The plunger is a small metal piece with a round rubber base. Rinse it off if there is any dirt or debris. If it is worn or torn, replace it.
Reinsert the solenoid plunger into the housing. Screw the solenoid back into the sprinkler valve.
Turn on the water and test the sprinkler. If it is still not working, replace the solenoid.