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To add to the reply from @63johnw
To get the best advise the first thing to establish is if you are PAYG or pay monthly, this thread can then be moved to the correct board.
The general advise when being unable to log is to clear your cookies and PC cache, very often it is a cookies preventing log in. If this just the Application, try going into the Application settings and reset the application, followed by a reboot of the phone. In order for Vodafone to locate your account you will also need to use mobile data for the log in but will be able to use WiFi for the following log ins.
If you are just trying to add additional data to take care of the rest of the month, you can purchase additional data here: Vodafone Extras . The only thing to remember the extra will roll over on a month to month basis and if you no longer need the extra, you will need to make sure it is cancelled.
If you still have problems try live chat or Customer Services first or contact the Social Media Team here: Contact Us