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.
Some users have reported as many as 8 or 9 attempts were necessary before success.
After you have Recovered your Device...
Re-Sync your Content or Restore from the most recent Backup...
Restore from Backup > http://support.apple.com/kb/ht1766