Apple IPhone 5c does not allow incoming or outgoing calls
A couple of things to try:


Try forcing it off (hold power button and home button down for
10+ seconds) and back on.
Try removing and reseating the SIM. 
Make a test call to customer service (611 in the US).
If none of the above work, you can reset network settings (Settings -> General -> Reset -> Reset Network Settings).


This is what any tech support (Apple Store, carrier) would have you do first.