The "NO AC" display is a diagnostic message telling you that the controller has stopped receiving AC power. The controller is running on the power supplied by the backup battery.
Check the power source by plugging a lamp or radio into the wall outlet to verify there is power at the outlet. If your controller is not plugged into a wall outlet, the power source is hardwired.
If you do not have power to the outlet, check all wall switches, timers, or power strips that may be connected to the outlet. Have an electrician check your GFI, circuit breaker, and electrical outlet if you do not have power to the outlet.
To verify that you have power to the outlet, disconnect the backup battery from the controller. If the display goes blank after removing the battery, the transformer could be damaged and may need to be replaced. If you a have a mutli meter you could also try testing for 24VAC on the two AC terminals inside the controllers wiring compartment.
Note: Controllers that are hardwired to the power source will need to be tested by a qualified electrician.