If SQL Monitor is configured to authenticate users with Active Directory, and you're locked out of your Active Directory administrator account, e.g. if the administrator leaves the company, you need to manually add a new administrator to the SQL Monitor database.

For versions 11.1.11 or later:

https://documentation.red-gate.com/sm/troubleshooting/can-t-log-in-to-sql-monitor-with-active-directory/unable-to-access-administrator-account

For versions 11.1.10 or earlier:

https://documentation.red-gate.com/sm10/troubleshooting/can-t-log-in-to-sql-monitor-with-active-directory/unable-to-access-administrator-account