It may have said to do a Restore, not "reformat." If the iPod still appears in iTunes when connected, the Restore is done from the iPod's Summary screen in iTunes. There is a button there for Restore.
If the iPod does not currently appear in iTunes, try putting the iPod into Disk Mode
http://support.apple.com/kb/ht1363
If the iPod goes into Disk Mode, run iTunes on the Mac and connect the iPod. iTunes may prompt you to do a Restore, which you should do. If the cause of this problem is data corruption, not a hardware problem on the hard drive, the Restore should give the iPod a "fresh start." If should appear in iTunes after the Restore completes, where you can set it up to sync as before.
Here's more info about doing a Restore, if you need it.
http://support.apple.com/kb/ht1339