Try either or both of these things.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.
Quit the app store and restart. Go to the home screen first by tapping the home button. Quit/close open apps by double tapping the home button and the task bar will appear with all of you recent/open apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus sign in the upper left corner to close the apps. Restart the iPad. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
I don't think that this is going to work, but its easy and worth a try at this point.
Sign out of your account, restart the iPad and then sign in again.
Settings>Store>Apple ID. Tap your ID and sign out. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
Go back to Settings>Store>Sign in and then try again.
You may need to restore the device in order to get this to work again, but first things first.
When you restore your iDevice, you restore the iPad to its original, out of the box, factory settings. The last step in restoring the device is to restore your iPad from a backup in order to recover all of your app data and then you follow that with a sync with iTunes to put all of your apps, music, photos, and other media back onto the iPad. So when the complete operation is peromfed properly, you shouldn't lose anything. What you are doing is reinstalling the iOS because something may have gone corrupt.
I mention restoring as a possibility and am saying that it may be the answer to this problem after all else has failed. It may not work either so it would be a last resort - like I said, if all else fails. But there are still other things that you can try before that.
Read this support article from Apple and read all of the links that the article references so that you will have a better understanding of how the process works.