You can not unblock the sim as it is killed dead and you will need to wait for a replacement sim in the post
you blacklisted your handset but finally found it, please let us know and we'll unblock it for you. We'll need your handset's IMEI. Don't worry its easy to find, just dial *#06# and it will be displayed on your phone.
Send your IMEI number to us by contacting our agents here.
In order to have the handset / IMEI bar lifted, the replacement SIM must have been received and fully activated.
PLEASE NOTE: The handset / IMEI bar can take up to 5 working days to be removed by the escalations / tech team.
more info here https://community.giffgaff.com/t5/Solve-your-questions/I-ve-recovered-my-lost-stolen-phone-SIM-how-do-I-get-the-bar/ta-p/3487461