It does not appear that you have gone with 3 months inactivity since you first registered. You may have a damaged sim. Order a new one from the link below and do a sim swap when it arrives https://giffgaff.com/orders/mgm
It may be that the debit card is coming to the end of it's validity. I think it needs to have more than 1 month left in order for it to work. Check the expiry date of the card and if it is running out soon then that is the problem. You may still be able to buy a voucher, order a new card from the bank.