Could it be that you had DFS radar detects? If the AP detects radar signals on it's channel, it has to shutdown the 5GHz radio and move to a non-DFS channel (if exists) or wait some time for another channel to be clean.
Can you run from the CLI (or maintenance tab in the UI) the 'show ap arm history' command?
AP303H-1a:68# show ap arm history
Interface :wifi0
ARM History
-----------
Time of Change Old Channel New Channel Old Power New Power Reason Result
-------------- ----------- ----------- --------- --------- ------ ------
2020-09-24 17:46:53 153- 153- 18 15 P- Configured
2020-09-24 17:46:53 124+ 153- 18 18 R -
2020-09-24 07:32:52 52+ 124+ 18 18 R Configured
2020-09-23 11:17:40 100+ 52+ 18 18 R Configured
2020-09-21 05:28:46 116+ 100+ 18 18 R Configured
2020-09-20 21:15:03 124+ 116+ 18 18 R Configured
2020-09-20 17:18:56 116+ 124+ 18 18 R Configured
2020-09-20 15:03:54 132+ 116+ 18 18 R Configured
2020-09-18 17:51:54 132+ 132+ Max 18 P- Configured
Interface :wifi1
ARM History
-----------
Time of Change Old Channel New Channel Old Power New Power Reason Result
-------------- ----------- ----------- --------- --------- ------ ------
2020-09-18 17:52:09 1 1 Max 9 P- Configured
I: Interference, R: Radar detection, N: Noise exceeded, Q: Bad Channel Quality E: Error threshold exceeded, INV: Invalid Channel, G: Rogue AP Containment, M: Empty Channel, P+: Increase Power, P-: Decrease Power, 40INT: 40MHZ intol detected on 2.4G, NO40INT: 40MHz intol cleared on 2.4G, OFF(R): Turn off Radio due to Radar, OFF(MA): Turn off Radio due to Mode Aware, ON: Turn on Radio, Dynamic Bandwidth Switch, AIRMATCH: AirMatch Event, I*: CCA Interference, C: Radar cleared, NC: Noise Cleared, Random: Random Channel, RMC: Radio Mode Change, RCP: Radio Client Preference Change
From there you should be able to see what happened to the radio settings, and may see the why of it. Note that the 5GHz radio is radio 0. You can see a few radar detects in my example above (flag: R).
Grabbing a 'show tech-support' from the AP as soon as you detect the issue, and a second time as you find the recovery would help as well. It contains all relevant logs. The 'show log wireless' and/or 'show log system' should contain information on what happened as well.