Failure to sign in to Hotmail (rebranded to Outlook) is typically due to things such as entering an incorrect email address or password, typing with the Caps Lock activated, or a maintenance or outage problem. A hacker may have also gained unauthorized access to the account and changed the password.
Sign-in problems are often fixed by resetting the password. A complicated password composed of letters, numbers and symbols is recommended to prevent hackers from gaining entry to the account. If the problem persists, users can contact a Microsoft customer service representative and provide specific required information to verify ownership of the problem account.