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.