It happens pretty frequently that someone who was the only admin of some boards leaves the team associated with those boards. If you're referring to a Free Workspace and you're a Workspace member, you can upgrade your team to Trello Premium, which adds several features to make managing teams a bit easier, including the ability for Workspace administrators to take control of all boards in the Workspace. You can sign up for Premium here if that's the case, and there is typically a free trial offer that you can utilize if you've not used it before.
Another option if the user's associated email address is a company email address (e.g. jim@yourcompany.com), you probably have access to their inbox by way of your email management system administrator. You can trigger a password reset at https://id.atlassian.com/login/resetpassword and reset their password, then log into their Trello account. Then, make yourself admin over the boards in question.
Hopefully, that will help, but please let me know if you have any more questions or concerns!
Trello Support Team
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