Is there an airplane icon on status bar next to battery percentage, where normally signal bars should have been? If there is, it is airplane mode. You can disable it just by long pressing the power button and when a pop up appears tap on the airplane mode icon. Then your network will be enabled and signal bars will return. If you're referring to safe mode, a simple reboot will get your phone out of it.
Also if you're referring to Internet calls, you should enable data traffic from settings, wireless and networks, other, mobile networks, mobile data traffic.
