If the users have zero connectivity, then access and check the status and health of the access points (operating in standalone mode) providing coverage in the area affected.

If the APs are centrally managed by a controller, then their user interface (GUI) should provide ways to check the status of the APs, their health, and specifically the SSIDs they provide services for. From the following screenshot, we can see that a Cisco Wireless Controller reports the number of APs, their uptime, and so on.

Please be aware that there is a process for APs to discover the controller, join the wireless domain, and download the configurations/policies. I recommend referring to specific vendors' troubleshooting documents to diagnose and resolve the issues.

If the APs are missing from the controller's user interface, then there could be a connectivity issue between them. Troubleshooting connectivity issues between APs and the controller with packet captures is the same as between two PCs in a network.