Hi there @ioanacoscodaru
Sorry to hear about your phone/sim being lost/stolen ☹
If you wish to block the sim to prevent it being used then please use this link -https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Sim swap procedures are carried out when a sim card is lost (and you are replacing it WITHOUT REPORTING IT LOST as that is a different procedure. If you have lost you sim and want to quickly get back up and running WITHOUT reporting it lost, then feel free to go ahead and do the sim swap now), damaged or you need a different sized card.
Sims swaps transfer your number and all current credit/goodybag allowance to your new sim which you will be sent. OR to save time you can buy one for around £1 fom Tesco, Poundstretcher, Asda, B&M Bargains and other major stores OR if you already have a sim then you're ready to go ahead and proceed with the procedure.
Sim swaps DO NOT work between 21:30 and 04:30.