by Ken Task.
Check your mdl_role_assignments table. Note the user_id field corresponds to those users who have access to admin levels.
select * from `mdl_role_assignments` where `roleid` like "1"
Could be the initial user suggested by the original install (normally admin) wasn't used by the original installer but rather used something else OR changed it before leaving their support role. That account was supposed to be unique and could only be edited by logging on as that user - ie, others shouldn't have been able to delete that user/account.
Then check those users id numbers in mdl_user for login name, etc...
'spirit of sharing', Ken