Turn on the sprinkler system and allow it to run for 10 minutes. Locate any pooled water on the surface. This is likely the location of the leak.
Dig up the area around the flooded surface until you hit the PVC sprinkler line below. Be careful not to cut through the pipe with the shovel.
Inspect the line for signs of cracks or other damage.
Cut out the damaged area of the pipe using PVC cutters. Make your cut 1/2 inch from the outside of each end of the crack.
Place a PVC coupler over one end of the exposed pipe. Tighten the locking compression nut on that end of the coupler by hand.
Extend the repair coupler over the other end of the exposed pipe. Tighten that compression nut by hand.
Turn on the sprinkler and check the connection to make sure it's not still leaking. If it is, continue tightening the compression nuts by hand or use a pipe wrench until the leak stops.
Fill the hole up with the dirt that you removed and replace any turf on the surface.