Did you enter the wrong PIN three times? Then your handset is locked. Enter the PUK code and then set a new PIN.
Tip: Don't choose codes that are easy to guess, like 1111 or your year of birth. This way your handset will be more secure.
Do you get the message 'SIM card rejected'? Remove your SIM card from your handset for a moment and then put it back in your handset.
Perhaps you already entered an incorrect PUK code ten times? Then you will get the message 'SIM card rejected'. Your SIM card is no longer usable.