Go into Settings >> Cellular Data, and make sure it is turned on.
If it is, go ahead and try restarting the iPad. Hold down the power button for about three seconds, until the power off slider shows up. Slide it off and wait until it finishes shutting down. Then hold the power button again, about three seconds, until you see the Apple logo.
Give it a little time to reboot and find the network. Hopefully that does the job.