Lift the sprinkler head out of the ground. It is on a spring and should pull up easily.
Wipe off the sprinkler head with a damp rag to remove dirt and debris from it. This alone will often unclog the sprinkler head.
Insert a thin wire or the tip of a knife blade into the sprinkler head to dislodge any debris that is stuck inside the spigot of the sprinkler head. Be careful to not damage the head and to not cut yourself. Wear protective gloves if you are using a knife.
Twist the sprinkler head off if it is still clogged and drop it into a solution of warm, soapy water. You can also mix warm water with white vinegar in a half-and-half solution to help break up stubborn dirt from the sprinkler head. White vinegar is mildly acidic and breaks up dirt.
Rinse off the filter screen if your sprinkler head has one. Screw the head back in and test the system to see if the sprinkler is working again.