Have you tried rebooting your IPP? (Hold the power and home buttons simultaneously until it restarts)
If that doesn't work you might have to restore the IPP using iTunes. I was having problems with all the games on my IPP not having sound, even though the sound worked fine otherwise. Delete/redownload apps didn't work. Restarting (rebooting) IPP didn't work. Then I backed up the IPP and restored it (I took the drastic step of restring as a new iPad) and everything works fine.
Because I chose to restore as new and then reload everything onto the IPP de novo, I lost some PDFs I had imported into iBooks but I knew that would happen. You can restore from the backup and avoid this problem. I just wanted to be sure I had a completely fresh restart--kind of like doing a clean install when you update Mac OS X.