It was some time ago, so I unfortunately can't remember all the details. It occured during a normal system update that ran without problems at first, but when I rebooted the phone to finish the update it couldn't boot anymore (I can't remember the error messages). I don't know what exactly went wrong or was corrupted during the update. I definetly rebooted the phone multiple times and tried to fix the issue, but after some time I realized it was futile.

Maybe there still was a way to salvage it, but I really can't tell in hindsight.