If unluckely you entered 3 times a wrong PIN code, your SIM card will be blocked. This aims to protect it from unintended usage from others.
To unblock your SIM card, you need to use your PUK code (Personal Unblocking Key). You will find this PUK code on the plastic card, which was holding your SIM card. Please keep it in a safe place and not near your handset.
To unblock your SIM card, you just need to follow the instructions on your handset with the use of your PUK code.