This could be caused by a few things. One is that there may be a problem with your billing and your cell phone service provider has disabled your service. If you own a prepaid cell phone and you run out of minutes, the phone will also only allow emergency calls. It’s also possible that either you or someone who has been messing with your phone has adjusted the call restriction settings of the phone.
On many phones, it’s possible to restrict all incoming and outgoing calls while still providing the ability to make emergency calls. Yet another reason is that the phone may be locked and a password may need to be entered in order to unlock it. Many modern cell phones still allow emergency calls when the screen is locked.