you need to check with the network that the phone is connected or not barred, bills paid. clean the sim card and the sim slot. if problem persists and you have dropped phone etc it may be the antenna on the dock connector that is the problem.
if the phone has been opened at any stage the antenna prong on the top of the dock connector may not be making contact with the metal bezel of the iphone.
the final reason for no signal is that the phone is reported lost or stolen and has been barred by the network...