First, make sure that your SIM card is associated with an active account. This can be confirmed by calling the wireless carrier and providing the SIM ID (also known as ICCID) printed on the SIM card.
If the SIM card is associated with an active account and your phone is not detecting the SIM card, please remove and re-install it back into the phone. If the SIM card is still not detected, it could be damaged. Please visit your wireless carrier store to have the SIM card replaced.