Gone and blocked your SIM? This usually happens when the wrong PIN's entered too many times, but it's easy to get sorted.
You'll need either a PUK1 or PUK2 code (your phone will tell you which one). You can find this by opening the Three Pay As You Go app or logging in to your account and selecting Account security.
If you've tried to guess your PUK code, or entered it wrong over 10 times, your SIM may get blocked. If that happens, no worries – it's easy to order a replacement SIM.