I backed up my ipod. I then updated it but all of my data is still gone. How do I get my data back??

Were the app in your iTunes library? If not the app data in the backup has no place to go. Redownload the apps in your iTunes libaray

Downloading past purchases from the App Store, iBookstore, and iTunes Store

and then restore from backup.

iOS: How to back up and restore your content

Otherwise, try restoring from backup again.

If still problem that means the backup does not include the data.