Hi
@amaramughalAre you on the £20 goodybag? if so have you used up the first 6GB of data? As the £20 goodybag gives you 6GB of UK data at full 4G speeds and once you used up the 6GB then you will continue to get unlimited data allowance at a reduced speed (256kbps) from 8am to midnight. From midnight to 7.59am you have an unlimited data allowance at full speed.
If you have not used up the first 6GB of data or if your not on the £20 goodybag then i suggest you check if there are any problems with the mobile signal & mast issues in the area please see this link
http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/manual-roam/m-p/3345471#M32502Also please try reconnecting by switching "Flight mode" On and back Off ("Settings" > "Flight mode"). This resets the active connection.