If you have purchased the Ad Removal package for the Fongo App or Fongo World Edition, and you uninstall the app and then reinstall it, you must restore the purchase from the 'Add-Ons' screen to remove ads again.
If the 'Restore' button does not appear in your app, you may have disabled 'In-App Purchases'.
To re-enable 'In-App Purchases', you can do this through the Restrictions Settings
Go to Settings -> General -> Restrictions
Then turn on 'In-App Purchases'.
Your ad-removal option should now be reactivated, and it should no longer be purchasable in the 'Add-Ons' screen.