You turned on the main line, opened the valves in the backflow, still no water to the backflow?
Could you have a master valve before the backflow?
Another ball valve/cut off in the main line?
To check for master valve, open your controller. It might be as simple as having a wire going to a connection labeled MV or PUMP or even PUMP/MV. If so, you have one. It may not be working.
If not sure, you should have one common, usually white, and 1 wire for each zone. If you have zones +1 wire you may have a master valve. In other words, if you have 7 zones but 8 control wires, what does the other one go to? Possibly master valve.