Thank you for contacting the GiffGaff Community. Hopefully the following information will help to assist you further.
It could be that the card has been blocked due to unsuccessful payments or other reason... As we in the forums are just Members like yourself, we cannot action any direct Account Related issues, therefore, please make contact with a GiffGaff Agent who should be able to help you with this isssue. You can do so via this link : https://support2.giffgaff.com/app/ask/ and choose an appropriate topic or one which is as close to your issue that you can find.
Please also ensure that you give the GiffGaff Agent as much information as possible in your message to them so that they can assist with resolving things for you more quickly, if your message is vague and doesn't contain all the facts then this will take them much longer to address things and sort out your problem.