Try deleting all your card details and reinputting them afresh.
Try clearing your browser's cache or another browser.
Also if you are using the mobile site https://giffgaff.com/mobile
try the full site http://giffgaff.com
and vice versa.
Finally try using another device, pc, laptop or tablet.
If no joy, then you will need to ask an agent for help here http://support.giffgaff.com/app/ask
Or get a gg voucher, they always work for me.