If you can’t log in to portal.office.com, several factors might be causing the issue. First, ensure your internet connection is stable. Double-check that your username and password are correct, as incorrect credentials are a common problem. If you've forgotten your password, use the "Forgot my password" link to reset it. Your account might also be locked due to multiple failed attempts or inactivity. Additionally, your organization’s IT admin might have restricted access or disabled your account. Ensure your browser is updated, clear cookies and cache, and try logging in again. For persistent issues, contact your IT support or Microsoft support.