Reset - press & hold the 'Power' and 'Home' buttons simultaneously until the Apple logo appears - this may take a longer time, so have patience; the iPad will 're-boot' - NOTE that none of these maneuvers will alter your apps, files, or settings.
Look at Settings - General - Restrictions. Perhaps it's disabled there.My App Store icon is nowhere to found since IOS updating! How can I recover it? There could be several other App missing too, that I have yet to discover. Not a happy chappie just now