Not enough information. Are you at a location where you know you have had service before? Are there any outages in your carrier's network (check the carrier's site)? Did you activate the phone again?
I suggest you try turning on airplane mode and than turning airplane mode back of a few seconds later, that trick does wonders.
if that doesn't work hold down the power+home button together until the apple logo appears on the screen (will hard restart the phone).
otherwise try restoring again.