Hi Ann,
Welcome to Atlassian Community!
The most common reason for this is that your account was using a different email address. When you've logged in again you've used a different email address and are now in a different account. I'd recommend sending a message to our support team with a link to a board that you can't access anymore. They'll be able to provide you with some information about how to access this again. You can contact support here: https://trello.com/contact.
Just as an FYI, if you have deleted your boards, even if it was accidentally, we cannot recover them. But, if it's just associated with a different address. You'd need to login again.
Do let me know if you've more questions. I'd be happy to help.
Cheers,
Vishaal S | The Trello Team
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