Make sure you have the Latest Version of iTunes (v11.1.5) Installed on your computer
- iTunes free download from www.itunes.com/download
Then... See Here > http://support.apple.com/kb/HT1808
You may need to try this More than Once... Be sure to Follow ALL the Steps...
Take your time... Pay particular attention to Steps 3 and 4.
After you have Recovered your Device... Re-Sync your Content