Two things can cause this:
Most often it's using the wrong login info. When you log into Office 365, make sure you are using your Park ID number followed by @park.edu. For example, #######@park.edu. Then use your regular Park password.
If you are using the right password and it still doesn't let you login, the syncing of your account may be off between Microsoft Office 365 and Park. To fix this, go to the password manager at
Log in with your current Park ID and password, and reset it there to actively re-sync your credentials with Office 365. The re-sync should happen within 30 minutes of resetting your password. Then return to http://login.microsoftonline.com to log into Office 365.
If after 30 minutes you are still unable to log in, please use the Submit A New Ticket button so an Administrator can be assigned to investigate.