Turn off FaceTime and sign out of your ID in Settings>iTunes & App Stores (or in Settings>Store)>Apple ID. Tap your ID and sign out. Restart the iPad and then go back and sign in again - but sign in from the FaceTime Settings.
if this works, go back to the store settings and make sure that you are signed there as well.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
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