Try each of these things and see if they work.
1) Turn your phone off for 5 minutes and then restart. This means powering it down completely, not just turning the screen off.
2) Check for signal problems or maintenance with your local mast. You can do that here -
3) Try doing a manual roam. This resets your phone's connection to the network. You can find instructions for doing this here -
If you're still having problems, try taking your SIM out and popping it into a different phone and trying another SIM (from a different provider) in your phone. This will help to determine if the problem lies with your phone or the SIM.
Hope that helps
Hi Stu, assuming you have Airtime Credit or a goodybag to cover the cost of calls/texts, can I suggest that you start by checking the giffgaff network status in your area to see if there are any local issues. http://www.bit.ly/ggservicestatus
If the signal is fine, you can try manual roaming by following the link to guide below http://community.giffgaff.com/t5/giffgaff-Top-Tips/manual-roam/td-p/3345471
That can sort all types of issues!
Would also do no harm to reboot your mobile, if not already tried.