If AP looses connection to ZD controller then current behaviour is that AP will continue to service existing users until it reboots after user defined time. Time between Connection to ZD loss and AP reboots no new connection is accepted.
After AP reboots, ZD is NOT found then WLAN interface is down hence no service is available.
i think in your case you can shorten the time for AP to reboot.
You can look at that setting at ZD GUI ---- configure ---- Access Points -- AP policies --- auto recovery. Hope this helps.