Hello Lisa & welcome to the giffgaff Community
You should probably check the service status in your area by following this link http://www.bit.ly/ggservicestatus
and entering your postcode. This should give you display any local issues, including planned work, as well as indicating signal strength (2G & 3G) & outages.
If you find that there are no issues with the signal you can try manual roaming- http://tiny.cc/243r0w