Hi there, 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.