@DaveJ - A common situation is that someone creates a Free Plan team, then invites someone to edit a board in that said, e.g., you, at which point you become a member of that team. Then, the new team member creates a board in that team. And later, a Team Admin removes you from the team. Now you can no longer access the board.

Here is a post where I replied to a similar situation - hopefully it leads you to a resolution:

https://community.miro.com/ask-the-community-45/lost-my-board-free-plan-no-link-11907