You cannot see the System tab -> User Roles page when logging in as a Master Admin. You have no other roles with this access, so you are locked out of the roles page and cannot correct this issue.
Someone has modified the role permissions for the Master Admin and removed this page for the role permissions.
This requires a change to the database. If you are confident running SQL queries, see below. If you are not happy doing this, please raise a support ticket and reference this KB article.
DELETE FROM funcaccess WHERE admingroupid=2
Be cautious when using this SQL query as it deletes records