There is no SIM card in your phone or you may have inserted it incorrectly.
The SIM card connectors need cleaning. If the card is damaged, contact your network operator.
Your phone is set to work only with certain SIM cards. Check if you are using the correct operator SIM card.
You have entered your PIN or PIN2 incorrectly.
Enter the correct PIN or PIN2 and select OK.
You have entered your PIN or PIN2 code incorrectly three times in a row.
To unblock, see SIM protection.
Codes that you have entered do not match. When you want to change a security code, for example your PIN, you have to confirm the new code. See SIM protection.
You have entered your personal unblocking key code (PUK) incorrectly 10 times in a row.