Hi Im sorry to hear your sim card
Please follow this link http://giffgaff.com/support/lost
to report your SIM/Phone lost/stolen.
Once reported giffgaff will send out a replacement SIM first class to the address found under your details page here http://www.giffgaff.com/profile/details
. The replacement SIM will have still have your current number , any active allowances .e g bundle(goodybag/gigabag) and/or airtime credit. when it arrives you will need to activate it online here http://giffgaff.com/activate-sim
Hope this helps you