If you live in a freezing climate, your pipe probably cracked, of there is still some ice that didn't get blown out during the winterization and is still not yet thawed out. Wait a few days or a week or so and start it again. If that doesn't work, dig from the head thats working to the one that isn't working and replace the pipe instead of trying to figure out what's in there. Your sprinkler riser may have some type of little critter that got in there but can't get out, may be caugt in your riser or your pipe somewhere.
