Access to iTune Access Issues

If you changed your password but still get an error message, try these steps to clear the login block:

• Verify on the Web: Log in directly at Apple ID Management to ensure your account is active and not locked for security reasons. 
• Check Date and Time: Ensure your computer or phone's date, time, and time zone are set automatically. Incorrect time settings are a primary cause of authentication failures. 
• Sign Out and Back In: Sign out of your account in your device settings or iTunes/Media & Purchases, restart the device, and log back in with your new password. 
• Two-Factor Authentication: If prompted for a 6-digit verification code, make sure you enter it immediately after your password or append the code directly to the end of your password if the field requires it. 

Apple seem to have changed something on their side which has broken authentication for many users in iTunes for Windows.