I had this problem and have had to order a replacement sim.
I have found Giffgaff agents are not particularly usefu.
I work in the industry and have some degree of technical knowledge as well as enough common sense to get by.
If there is an issue which requires any more than the above you will be dissappointed.
This was the cause of my problem - hope someone finds this useful.
O2 Network issue
Please be aware that O2 are currently experiencing technical issues which is affecting the UK as well as users abroad. Some customers will experience a message stating “sim registration failed” displayed on their handset or low/zero coverage. Other customers may experience dropping calls. O2 Network services are working on this but as yet we have not been advised of an ETA for resolving this issue.
Too late for that mate, already barred my sim and ordered a replacement, have to wait with no incoming calls on my main line until the replacement arrives now.
there is a thread on this
a reprovision of your sim would have sorted it
I've been without service since early yesterday morning.
I did a search and found nothing on the subject so I just assumed that my sim had gone faulty (unusual i know but it can happen) by the time I got any relevant information I had already barred my sim and ordered a replacement - I'm also being told that I have to wait for the replacement to arrive and they cant provision another giffgaff sim which I have in my posession with my mobile number.
This has been a complete disaster....