If these don't work, can you get o2 coverage in your area? - webmap.o2.co.uk
Or is maintenance happening in your area? - status.o2.co.uk
If no, you most likely have a data ban, ask an agent about this!
I think it would save people a lot of time if newcomers were emailed this by giffgaff upon activating their sim as it is becoming the most popular question on the 'help' board yet it is so simple. IT is taking up messangers time on the board when harder questions should be being answered.