Turn off the water in the house and also the sprinkler system if it is on a seperate branch. Go out to the water meter and watch it to see if it is moving. If it is, then you've got a burst line.
My main burst about 3 years ago and we had the same issue with a pressure drop, but all throughout the house. We lost over 25,000 before we were able to get it fixed. The h2o company did give us a partial credit.
If you do need a new main line, make sure you see that it is buried to the correct depth. I didn't watch the guys who repaired mine, but then we had a new paver walkway put in a couple of years later andthe landscapers found the water line was buried only 6" deep!