Why did this happen by itself?
You'd need to dig through the history to find out which admin did it (I'm including your admins and the Atlassian system admin in that group of admins).
That needs to be looked at by Atlassian support, as it needs someone to look at logs etc. I'd raise it over at https://support.atlassian.com and specify the user and any known dates (last time they were known to get in ok, first time you noticed they'd been disabled)
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