Error 21 problems are common, and are easy to fix. Just don't do what you did to cause the Error 21. If you get the Error 21 just by turning your Xbox on, well, then you may have a problem. See if you can still boot a game off of a disc, and if you can, then the Xbox should still be able to be fixed just by writing files to the hard drive: You shouldn't even need to open the Xbox if you're using softmod techniques.

Here is what information I have picked up about other Error Screens:

Error Codes

• 5: Hard Drive Unlocked "Error5 means you're: A) Booting the MS Bios, and B) MS Bios is complaining that the drive is unlocked."
• 6: Error Unlocking Hard Drive. (Example: Locked hard drive with invalid key.) "Results from either a mistyped key when using hdlock, or a bad key given by the wrong EEPROM" "for use in another xbox"
• 7: Hard Drive Unresponsive. (If you have been hearing clicking on the drive, that likely means a hardware failure. Otherwise, make sure the hard drive is plugged in correctly.)
• 13: I believe this means Dashboard Error. Either the dashboard cannot be found, or a new kernel is trying to run an old dashboard
• 16: unknown. Seems to have something to do with a dashboard. Try placing the original dashboard files back where they should be. Invalid dash (such as when the clock must be set, but the dash cannot be loaded... Perhaps this is why some Errors will turn in Error 16.) (some unknown link I saved earlier)
• 21: Unable to run an Xbox executable. Perhaps signature checking noticed an unauthorized signature (probably the most common reason of any of these error screens is this), or perhaps the file trying to be executed does not exist.