Did you perform a SIM Swap? If so, the SIM Swap process may take anything from a few minutes to 24 hours to complete.
However, it is normal completed with 30 minutes. Please turn your phone off/on again to see if that helps.
If it's been more than 24 hours the SIM Swap has not completed successfully, please contact a Giffgaff agent to look into this.
To contact a giffgaff agent you will need to fill in the agent form here https://giffgaff.com/support/questions