I've been in my house for about 2 years now, and just yesterday my alarm went to "no ac" and lights on the ready or armed are not there. The panel is still lit and the motion sensors are active. This morning I messed with it again and no longer is " no ac" there.
"AC is now showing" not ready is being displayed in the screen. How do I reset it? Do I have to do a new code? Please assist, and it's a Honeywell, not sure of model#.

Check your circuit breaker to this panel. No AC could mean no alternating current rather than air conditioning. I think you popped a breaker.

Your panel gets its power from a 16.5? Transformer, and if power goes out to the receptacle or transformer fails, "no ac" will show, when restored, the indication should go out. The battery will take over and continue taking care of system until battery is drained.
Is this your system?