This is a tricky one, as I’m not entirely sure how you’ve done it. 
But this is for all users, not just admin? (to rule out the use admin ip allowlist setting)
Do you have a CDN in front of your forum? If you disabled a user by deleting them, and then clicked the ‘Block IP’ button, that may account for the unintended blanket lock-out. If so, you could try disabling that.
If you have server access (and are comfortable) you could look at the IP blocklist using the rails console, and delete the one that’s jamming you up: