Poor Network / No Signal Received
Have a read of http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/Getting-Checking-Signal/td-p/3764957
O2 Status and Coverage
Check the O2 status and coverage:
O2 status: http://bit.ly/o2status
O2 coverage: http://bit.ly/o2coverage
A quick restart and/or a reboot of the device can help you get reception on your phone. If these don't help then sometimes you may need to restore the device to factory settings (how it was when you took it out of the box) but then you will need to reinput your internet settings.
Turning the device into airplane/offline mode and back into normal mode can, a lot of the time, get you a bar or two of reception in bad coverage areas but also full reception if it just a temporary network issue!