Oh no, @LushLife I'm sorry you had to switch out your Three SIM to begin with, that's not great to hear. It's extra annoying that the card has now gone missing, they are tricky to keep track of!
To make sure your old SIM card isn't used by anyone else, the first thing to do is report it as lost. I've sent you a PM with more information on how to do that. You can request a new SIM at the same time.
Alternatively, once your old SIM is reported as lost, you can pop your details into this form to request a replacement. We'll send that out to your registered address via first class mail. If you can visit your local Three store with valid photo ID instead, you can pick up a replacement there and then. Find your closest store using our store locator.
Hope this helps! We're glad to have you here on Three Community and hope you have your SIM up and running again soon.
StephR
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