Please Report your sim as lost/stolen here: http://giffgaff.com/support/lost
Giffgaff will send you a replacement sim within 24 hours. As soon as you report your sim lost/stolen, your sim will be blocked and cannot be used. All Giffgaff sims are now dual and hybrid sims that contain a micro sim which easily pops out.
All your account details, number, credit and any goodybag you have will be restored onto the replacement sim.