If you are trying to login to one of the Library e-resources, please work through the troubleshooting steps on this page - https://library.sydney.edu.au/databases/access-eresources-and-help.html or else, when you try to login and get the "Authentication Unsuccessful" page, scroll down a little and read the troubleshooting there.
The main issues are to not use Autocomplete in your browser, and to try synchronising your UniKey password, then changing it if that doesn't help. You need to re-synchronise your password even if you can still login to other services like Uni email, with your UniKey.
For students, there can also be various issues to do with your enrollment and your Library record that can affect your ability to login. You do need to have a currrent Library record, and you can check that by logging into MyLoans - http://opac.library.usyd.edu.au/patroninfo
If you've only just enrolled and you haven't completed your enrollment by picking up your Campus Card, you'll need to do this before your Library record is created.