PUK is an abbreviation for Personal Unblocking Key; your PUK code is an 8-digit code that unlocks a barred phone. If you have set a PIN password on your phone and then enter it wrong three times, you won't be able to use your phone again until you enter the PUK code. When you can't remember your PIN or accidentally enter the wrong code, it can be very frustrating. However, there are three ways you can find your PUK code and unlock your handset.
1. Look in your SIM card wallet. Most network providers will give you your PUK code with the SIM card holder and instruction manual when you buy the phone. Others send the code through the post in your welcome pack.
2. Contact your network provider's customer service team. If you can't find your PUK code in your SIM card wallet, contact your network provider. It will confirm that you are the account holder and provide you with your PUK code. However, remember you will need another phone, such as a land line, to do this.
3. Visit the network service provider's website, where some networks will allow you to access your PUK code online. Networks such as AT&T require customers to sign into their online account before accessing their PUK code. However, others simply require you to enter your phone number before displaying your PUK code.
And remember: Once you have unblocked your phone, you'll have an opportunity to reset your SIM code. Try to pick something memorable, such as the last four digits of your phone number.
For better understanding here is a picture where you can find the PUK code.
Hope it was helpful .