If you have not done so recently try restarting the iPad, and if that does not work a reset.
If the problem persists you might try calling the carrier. I just received a carrier update on my iPhone. It might be something like that, and I'm not sure how the iPad receives these updates.
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.