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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.
 
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