Try signing out of your iTunes Account in Settings, rebooting and then signing back in. Fixed my problem with iTunes on my iPhone
Logging out then back in didn't help me. :-(
Specifically what are you doing and I'll try to reproduce it. I've been searching and looking at apps and reviews but haven't bought anything yet.
Edit: And when all else fails, turn it off and on (the phone).I assume you've closed the app at least once during your attempts?
I tried rebooting, it didn't work.
I tried logging off then back on, it didn't work.
I logged out, rebooted, opened the iTunes Store app, then logged back in. Bingo!
Go to your settings. Log Out of iTunes Store and App Store. Then open the iTunes Store app. Then log back in again. Should be good to go.
The third thing I tried worked. That's why I said "bingo".