It may be the walls are to thick to get a signal you should all so try the sim card in a new phone to make sure it is not the phone that can not pick up a signal inside
But Try a Restart/Reboot/Reset, A quick restart and/or a reboot of the device can help you get reception on your phone. Also you could remove your sim and give it a wipe with a clean cloth, then reinsert it and make sure mobile data is turned on in the phone settings
Check your Internet Settings are right herehttp://community.giffgaff.com/t5/giffgaff-Top-Tips/manual-roam/td-p/3345471
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!