First off, count the number of Sprinkler Valves you have. Secondly, operate each one of them manually. Determine what part of the yard each waters. If there are 5 valves and all 5 water the entire yard, you have 5 zones and no master valve.
If, you cannot get water to any areas, unless you open one particular valve in conjunction with the others, then that valve is the master valve. The Master Valves wires would need to be connected to the Common and the MV Terminals of the clock, this will allow the clock to operate 2 valves at once, the Master Valve as well as you zone of choice. The Master Valve is to help prevent an individual valve from sticking on, causing a high water bill and/or flooding. Should a zone valve stick on, it would be able to run for the duration of the controllers program because the Master Valve
terminal is active until the program ends. If your having a problem with and individual valve sticking on, you'll need to check the solenoid, the rubber diaphram, the valve seat - (where the diaphrams rubber seat meets the valve body) - for debris, rips or tears. You could also check the clock to insure there's not a problem with the circuit board, allowing a constant 24 volts to a terminal post even when the clocks in the off position. You can easily do this by taking a working solenoid to the clock, make sure the clocks in the off position, then touch one of the solenoids wires to the common post and the other to each of the individual zones posts. If you get the solenoid to click, with the clock off, than that post is receiving 24 volts when it shouldn't. Replace Clock.....