What is the best case scenario if one can not login successfully knowing full well that the username and password are absolutely correct?

I am not sure how to answer this because it depends on what the username and password are for. If you are referring to a student's NetID username and password they should be sent to the Learning Commons and Technical Support Desk for that. They can help people having trouble with their IT account. If a student is having trouble with a different kind of username and password they can also try that desk and depending on the problem the staff at the desk may be able to help.