The "NO AC" or "Blank" display is a diagnostic message telling you that the controller has stopped receiving AC power. For the "NO AC" message, the controller is most likely 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.
Caution: High voltage testing on the transformers primary power side should only be done by a professional electrician or irrigation contractor.