Hi @MandyK — welcome to Figma Forum, and sorry to keep you waiting on this.
I've flagged ticket 2053831 to the Customer Support team and shared this post so they have the full context. They'll follow up at the email tied to your account with next steps.
On accessing the board itself: If it now sits in a folder with no owner and you're a team owner or admin, you may be able to reclaim it directly by opening the folder and selecting Join as Owner — here's the walkthrough: Claim ownership of a folder. Because the original owner is on a deactivated SSO login, Customer Support can confirm whether that route works for your setup or whether reassigning ownership is an option.
If it’s helpful in the future, Transfer ownership of a team walks through the options when an owner leaves, though the account-recovery route there depends on logging into the owner's account, which won't apply while the email is deactivated.
Please let me know if this helped in the meantime, and appreciate your patience as the team follows up.