The common cause of the Vodafone post-paid and pre-paid services not returning any data usage is due to authentication issues, such as the account being locked because of too many failed login attempts.
This can also affect any other accounts that have been setup correctly, i.e. they will no longer display any usage whilst the account is locked.
To avoid the account being locked, check you have entered your "Username" and "Password" correctly, ensure your keypad is not auto-completing/changing characters on entry into myUsage.
Note: If you have a PIN for your account, you may need to enter this in the "PIN (optional)" field to view any additional mobile services/account details on the Vodafone site.