When you open The Digi Diary url then it will be open User Panel URL http://yourdomain/site/userlogin (if Front CMS isnot enabled). By default The Digi Diary opens User Panel instead of Admin Panel (due to security reasons). 

In The Digi Diary your Admin Panel login url is http://yourdomain/site/login
Here superadmin, admin, teacher, accountant or any staff can be login through their email id and password.

And User Panel login is http://yourdomain/site/userlogin
Here only student and parent can login through their username and password.

Please check you are login with right panel with right credentials.