You have had this giffgaff username since 2017.
You can only activate a sim once when you join, and never activate it again.
You can not activate a sim that has been deactivated.
If a sim has not been used for 6 months to make any chargeable calls or texts then it will be deactivated, you will loose the number and the balance and there is no way to get it back.
Is this what has happened to you?
When did you last make a call from that sim?
If it has been deactivated because you did not use it then you need to get a new sim and join again as a new member and get a new number and pay again.