REBOOTING YOUR PHONE COULD HELP, turn it off for 1 minute then on again. Failing to solve your problem you can check nearby signal masts in case there's a problem.
Check for any disruption in services in your area
Calls are not working
If there's no credit on your pay as you go card, you can't make voice or data calls.
Top up your pay as you go card https://giffgaff.com/top-up
_____IF STILL NOT RESOLVED CONTACT AN AGENT THEY WILL SORT IT____
Direct link to ask an agent
EXPLAIN YOUR PROBLEM in box provided an AGENT will check for you
Agents are available from 8am till 10pm 7 days a week.