It's best if you contact an Agent and they should not only be able fix the problem but also if required do a refund,
They usually reply within 24 hours but don't be tempted to ask twice in a 24 hour period or the system may kick the request back.
Whilst you wait,
Make sure the Phone's Data connection IS enabled and if the phone has a,"Do Not Disturb" option that this is NOT enabled.
You might need to refresh your Phone's Settings/Network/APN settings.Try these simplfied ones.
Connection Name: giffgaff
APN - Access Point Name: giffgaff.com
Login - Username: "giffgaff" for all devices or "vertigo" for iPhones and iPads
Password: the password is "password"
Proxy: leave blank
MMS proxy: 184.108.40.206
MMS port: 8080
But don't forget to save them and then reboot the phone.