That usually means the static ips or vlan for dhcp you've assigned them are not working properly.
Something has changed somewhere, without knowing more about your setup it's hard to say precisely. I'd check your DHCP server and make sure it's up for the relevant vlan or make sure some uplink somewhere hasn't been changed and a vlan not passing through. You can test using a laptop and see if it can get online using the settings you'd be using for the APs as well
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