There are a variety of problems that can crop up in your sprinkler system, and, if you want your system to work properly, you'll have to take care of them. Some, more complex issues (say, with an automatic system) may require professional assistance, but here are some a few problems that have fairly simple fixes.
Repairing Broken Water Lines
A sudden drop in water pressure might indicate that one of your sprinkler system's water lines has been damaged. The first thing you'll need to do is locate the broken line. Look for wet spots or puddles in your yard (this is probably how you found the problem in the first place). A sudden drop in pressure is another sign of a leak.
Once the damaged line is located, turn off the sprinkler system at the main valve. Call up your local power company to find out if there are any underground utility lines you should be aware of. Then, dig out the area around the broken water line. Dig deep enough for you to be able to work around and under the pipeline (approximately 4 inches below the pipe). You should uncover about one foot of the pipeline on either side of the leak.
Use a hacksaw to cut out the broken section of pipe, doing your best to make a smooth, straight cut. Use PVC glue to reconnect the cut pipeline. Then, fit an expansion coupling over the reconnected pipe (while the PVC glue is still wet). Turn the coupling slightly, and then hold it place for a few seconds. Allow the glue to dry for about 15 minutes before turning the water back on. You can refill the soil after making sure the pipe has stopped leaking and that the new connection is water tight.
Replacing a Sprinkler Head or Riser
Replacing a sprinkler head is a pretty simple process. First, dig out the broken sprinkler head. You should make about a 6-inch circle around it. This will give you some room to work and will help to keep dirt from falling into the exposed sprinkler line. Unscrew the sprinkler head, being careful to avoid pulling the riser out of the sprinkler head. Sometimes, it's best to use pliers to hold the riser in place while you unscrew the sprinkler head. Then, simply screw the new sprinkler head into place. As you go about this repair, make sure that no dirt falls into the sprinkler line. Test out the new sprinkler head and, if it works, fill in the dirt around it.
The process is pretty similar when replacing a riser. Repeat the steps above for removal of the sprinkler head. Then, remove and replace the riser (You may need a nipple extractor to get the riser out). Test the system out to be sure it works before filling in the dirt around the sprinkler head.