Please explain if these are 'native' apps that come w/ the iPad or only the apps that you have added from the Apple App store (or both)?
Also make sure (especially if you have loaded on a LOT of apps) that the ones in question have not 'moved' to additional home pages - a home page can hold just 20 apps unless 'folders' are made which make apps less noticeable?
Finally, if none of the above helps and you've not yet re-loaded the apps as per Gabriel's suggestion, then you might want to try a 'Restart' and if not successful a 'Reset' of the iPad - Directions HERE - good luck & please post back your results!