Hi If you got a replacement SIM it will arrive with your current number and any active goodybag or airtime credit you have when your new SIM arrives you’re advised to activate it online immediately by clicking this link http://giffgaff.com/activate-sim ).
Contact an agent here https://support2.giffgaff.com/app/ask/SIM-and-number/Activating-my-replacement-SIM/form/Activating a replacement SIM
Allow up to 24 hrs for a response. Agents work 7 days a week from 8am to 11pm.
Check for replies here:https://giffgaff.com/support/questions
If you did a sim swap
Turn you phone off and on a few times to see if this will help your problem.
Hope this helps you