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:
*Standard international calling rates apply
Once you have your PUK code, enter your PUK code on your mobile as you would a phone number.
The formula to follow is: * * 05 * PUK code * new PIN * new PIN #
So, on your mobile phone's keypad enter:
star, star, 0, 5, star, 'PUK code', star, '1-8 digit number', star, 'repeat 1-8 digit number', hash
Please note: If an incorrect code is entered too many times, the SIM card will be permanently blocked (between 7-10 attempts depending on handset).
Here's a quick video on finding your PUK code in My Vodafone.