The most likely cause of not seeing your purchases in the iTunes Store is being logged out, or logged in with the wrong Apple ID. Or a glitch in your log in credentials.
Try going to the bottom of the first tab in the iTunes Store and looking for your Apple ID, or the sign-in. It should be in a box at the bottom left.
If signed out, sign in. Otherwise try tapping on it, signing out, then signing back in with your Apple ID.
You can also do this in Settings > iTunes & App Store.
Good luck.
Added later:
Keep in mind that your iTunes purchases are tied to the Apple ID you used to purchase them. If you've ever used a different Apple ID, or you have more than one person (past or present) who used the iPad with a different Apple ID then the missing purchases may be under that account.