Hi,
Contact the Customer Care section of your mobile phone service provider, give them your phone number and once they are sure it is really you, they will give you the PUK code, which you need to enter into your phone. You will then be asked to set a new PIN code. Once you have your PUK then keep it in a safe place away from your mobile as it is the key to unblocking your SIM.
Note that if you enter a wrong PUK 10 times in a row, the SIM will be permanently blocked, and you’ll need to purchase a new SIM card.