You should report your lost phone / sim to giffgaff as soon as possible, you can use the link belowhttp://giffgaff.com/support/lost
you will be given the choice to either "Bar SIM only" or "Bar SIM and Handset" so please ensure you only click on the "Right Option".
when you have reported the sim stolen/lost an agents will be able to pause any goodybag you currently have active until you receive the replacement Sim,
The replacement sim which will have your remaining credit/goodybag balance and more importantly, your existing giffgaff mobile number
when your new sim arrives you will need to contact an agent to get the replacement Sim unblocked. https://giffgaff.com/support/ask
activation can take upto 24 hours