Every irrigation has a common wire that connects all of the solenoids together and in most cases it is a black wire, as this is the colour most commonly used.
The common wire is found in the ‘c’ terminal and is vital to the systems operation.
If a common wire gets cut then all of the solenoids that it connects together will be out of action.
Its usual to have one strand of multi core wire serving the front of a house and another strand serving the rear, so if you find that all of your rear solenoids have stopped working then chances are your common wire will have been cut or broken.
From there it just a matter of tracking it and repairing it.