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.
Hello friends! From the community that brought you Welcome Wednesday, Throwback Thursday and Friday Fun, welcome to Taco Tuesday, a weekly discussion about all things Trello. The best part? One Tac...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events