The most common reason for login credentials to fail is because the account has been locked due to too many unsuccessful login attempts. This can be undone by the account owner contacting firstname.lastname@example.org or calling 1800 155 288.
How can I stop this from happening?
The most common reason for failed login attempts is shared logins. Ideally, you should never share your login details with anyone - if you need someone else to get access to your account, add them as a user or an admin so they can access your account using their own credentials. This will also give you better visibility of who's doing what on your account via the Activity log.
If you use a shared device to access your account, always remember to log out of your account each time you finish using it. You should also ensure you are logged out of any keychain accounts if you use them, as this may also mean that other users of your device have access to your passwords.
What other measures can I take to protect my account?
This article details a variety of security measures you can take to keep your account secure.