If your display indicates NO AC (see picture) indicates the control panel is not being powered by the normal 24v AC power.
1) Panel door is open
2) Fuse if blown in controller
3) Transformer is not working
4) Power source (*oulter or breaker) is not functioning
While the NO AC is displayed, the controller is on 9V battery power and will not operate zones or run your system.
If you are unsure on how to diagnose properly, please call a professional irrigation contractor to troubleshoot for you.