I am sorry you are having difficulties
Please check the signal status in your area to see if there are any known local issues, such as damage to a mast or maintenance being carried out, or poor coverage in your area. Remember, the status checker is only updated hourly, so recent faults may not yet show up – so check back after a while, just in case.
Simply enter your postcode in the following page : https://giffgaff.com/coverage-and-service
If there is nothing reported there you can attempt a manual roam to refresh your connection. http://is.gd/manualroam
And, of course, you can always try rebooting your phone as this often solves a multitude of problems.
You could also turn the phone off, take the sim card out and give it a gentle wipe with a soft cloth, before putting it back in and restarting the phone.