When you enter the wrong PIN for your mobile more than three times, your SIM card will be locked. This prevents anyone trying to guess your PIN. To unlock it, you’ll need a PIN Unlocking Key (PUK) code. You can find your PUK code on the package your SIM came in. If you don't have this anymore:
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Here's how to unlock your SIM card using a PUK code
Watch this quick video, which walks you through how to find and use your PUK code in My Vodafone.