If you enter a wrong PIN code too many times, usually three, your SIM will lock itself. This protects you from someone using your personal information or making calls if your cell phone is lost or stolen. You may see "PIN blocked" or Enter PUK code" on your cell phone display. The PUK code stands for PIN Unblocking Key. In the U.S., the PUK code is rarely provided when you get your subscription. If you do not have your PUK code, contact the Customer Care service of your cell phone company, 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.