@abbeybobo75 Was the former number a giffgaff number? Has it been used or topped up in the last 6 months? If not it will have deactivated And the number recycled.
You need to log into the username associated with that old number
If it HAS been used in the last six months or topped up, you ought to have done a sim swap, but by activating a NEW sim you have created a NEW account with a new username, if it is still active (the former giffgaff number) order a new sim or buy a sim from the shops that sells sims for £1 or less and back up contacts and do a sim swap https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
basically any UNACTIVATED giffgaff sim is suitable for sim swap.
Sim swap is not possible for a number that has been deactivated,