You have to add them back to the administration panel. Admin Panel>Users&Groups>Groups>Group Administration>The admin group should have a orange star next to it, edit the group and add it the accounts by name.
Are you the founder of the forum? I would suggest that if you are then you use the Forum Utilities in order to log back in to your admin panel. https://www.forumotion.com/utils. From what I can understand this is more of a password issue now. If you cannot log into the panel and your the forum's founder.
If the above does not work you will have to wait until someone else can help.
The founder email doesn't work anymore as it was removed by the email provider so I could create a back up founder email but I disabled the password reset to do so