You could try visiting your local O2 store to see if they can access your account, if they can they have the ability to transfer your number (and any credit on it) onto a new simcard. However you will need to pass a few security questions such as a frequently dialed O2 number, the balance on the phone, when you last topped up and how much you topped up by.
I work for O2 but my comments and opinions posted here are solely my own and do not reflect those of Telefonica.
