If you have a PIN on your SIM card as a security measure, each time you start your phone, you’ll need to enter it. If you get this wrong a number of times, you will lock you SIM card. This means you’ll be unable to connect to your network or use your phone.
To remedy this, you’ll need your PUK code to unlock the SIM. PUK stands for Pin Unlock Key, and it’s an 8 digit number. To get your code, you’ll have to call your mobile network provider. Enter this, and you’ll unlock your phone, and be able to use it again. Be careful not to enter the wrong code 10 times or the SIM will be permanently locked.