It could be one of two things:
The admin user has been removed from the admin group or the admin user has had administrator rights removed.
To fix this, you will need to do some manual database changes using whatever tools it supplies (like MySQL's Administrator tool and the like)
First, you need to verify the admin user's database user_id number. This should be 2 but can change. To do this, look at the data in your jforum_users table for the record with a username of Admin. E.g.:
Select * from jforum_users where username = "Admin"
Next you need to check if the admin user is still a member of the "Administration" group. First get the group_id from the jforums_group table with:
Select * from jforum_groups where group_name = "Administration"
Then check if the admin user is a member of this group by doing:
Select * from jforum_user_groups where group_id = <admin group_id>
If the admin user is a member,
you should see their user_id number in one of the records returned.
If the admin user is not a member, then you need to add them back in with:
insert into jforum_groups ( group_id, user_id) values ( <admin group_id>, <admin user_id>)
If the admin user IS a member, then you probably need to re-grant administrator rights to the group. You can do this by:
insert into jforum_roles ( group_id, name ) values ( <admin group_id>, 'perm_administration' )
Note: If you DB does not support autoincrement type key generation, you will need to include a valid role_id value as well, e.g. find the current maximum role_id value and add 1 to it.
[originally posted on jforum.net by monroe]