Some more detailed information could be found in the "Maintenance Guide"
Connect to the console port on the C25 and check whether the C25 boot to the rescue mode.
https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-reset-password-on-Identifi-Wireless...
Choose 1 for resuce mode - the C25 should boot with the factory installed image.
If you don't get to the rescue menu I think you'd need to RMA the unit.
Check the release note to see the min. SW to get to the current release of the WLAN controller.
i.e. min. version 08.32.01 to upgrade to 9.15