I haven't used trello for a few months. I sign in (I only have one account) and am now forced to 'sign in with Atlassian' or other services, and VOILA! ALL my boards have been wiped out as though I've never been here before.
Trello were acquired by Atlassian four years ago, so it wasn't that.
I suspect you mean the switch to Atlassian accounts has been applied (you were emailed about it, even if you didn't see the messaging on Trello's front end because you were not using it), and because you didn't do anything to update, it's defaulted to using your Atlassian account (the one you use here) instead of migrating to the email you were using for your old Trello login.
If it has not been too long, you should be able to recover your boards, but have a look at https://help.trello.com/article/1197-using-trello-with-an-atlassian-account to work through getting the right account associated.
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