Ensure you have the correct APN/Data settings. You need to set-up your handset with APN settings before you can use the internet or MMS with the giffgaff network.
As you haven't stated the mobile device you have, please follow the link below to find settings for the most popular handsets including iOS(iPhone, iPad), Android(LG, Samsung, HTC), Nokia Lumia(Windows) etc.
If you find the data settings are fine Please check the signal in your area, simply follow this link https://giffgaff.com/coverage-and-service to check either the network status or coverage. The network status should give you details of any local issues, including planned work, as well as showing signal strength (2G, 3G & 4G) & outages.
Please remember that the website is updated hourly and may not display any issues at the current moment.