You should be able to log in via MxAdmin unless there's a script in your application which removes it or someone changed it in the Project security to call it something other than MxAdmin.

What message do you get? Unknown user or something else – check the logs and it may give some more insight.

EDIT – If it’s a licensed project you can change the password using the Change Admin password button available inside the details of an environment:

If its  a sandbox I’d suggest you to add the admin (if not done) under Demo users and you can always check the password by clicking on the user and copying to to clipboard. You will need to redeploy the application if you add a new role.

Hope this helps!