This sounds a bit strange, maybe your default homepage is a custom login page?
What you could do is (temporarily) go to prototype/demo or production and enable demo users (make sure you have a demo admin account) and publish again.
If you see the role switcher when you go to your app, you can select the admin role and from that configure an admin user (of course you would need to have access to the user management overview for this).
If you cannot see the role switcher, you can copy the password and then log in with this account and create an admin user.