Try resetting the iPad see if it brings it back. Hold down the Home and Power Buttons until the Silver Apple Logo comes up. Ignore any sliders that pop up.
If that doesn't fix it, try taking out the sim card, and putting it back in. Making sure the the gold contatcs are clean.
If that still doesn't help you'll likely have to go to AT&T to get a new Sim, or ask them if there's a problem with their service.
So I go to AT&T. They put in a new sim card and still no service. The rep takes a known working sim card and installs it into my iPAD and it works. Replaces the sim with a new one and "No Service" again. The rep tells me to go back to Apple and ask them to check the provisioning on the Apple Servers. Does this sound right?