I would first check the ALERT BELL in the upper right-hand corner of the dashboard to see if it has a RED dot. If so, you can check the alert and it typically guides you through remediation steps.
If there are no alerts (i.e., it's already been dismissed) would look to see if all devices have power, are connected, are the configurations the same/changed (compare one AP to the other AP - if different then ask why - is it a different zone or placed in an area that needs to be configured different from the other APs on that partcular switch), see what event logs or indications of what might be causing the device to not come online as expected.
Sometimes "dismissing" an alert will bring everything back online without further changes or configuration.
As a last step, you could try a factory reset of the AP.
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