Sometimes, a little thing like a piece of bark mulch in a sprinkler fitting is enough to cause enough water loss that no heads on a zone will work because of the lack of pressure.
Tree roots not only destroy sprinkler heads but they will often pinch supply lines underground and cause a lot of frustration, trying to figure out why a specific zone is not working.
Sometimes, it’s not just a tree root causing problems with sprinkler lines but the entire tree!