I suggest you copy/paste the exact error messages you are getting to help make sure we are answering your question.
If I understand correctly, one of your admin accounts no longer has admin rights. So login with an account that has admin rights and add the other to the group "Admins". This group should have the permission "tiki_p_admin".
To manage users:
To enable features:
If you have no account that has admin rights, please see: Lost admin password