Why did you fsck with it if you don't understand what you're doing?
My guess is you borked the downgrade. Are you trying to upgrade from Mac or PC?
Try putting it in DFU mode, and restoring the desired firmware, from a NEW user account on your Mac/PC.
DFU mode:
http://iphoneindia.gyanin.com/2009/1...toring-iphone/
The "fix" for error 21
http://forums.macrumors.com/showthread.php?t=975858
http://discussions.apple.com/thread....ageID=11723151
Here let me google that for you:
http://www.google.com/search?q=iphon...pdate+error+21