iphone 5 won't reboot. keeps flashing in and out.

Hey Kash922,

Thanks for using Apple Support Communities.

Since your screen is flashing you may want to look at the hardware troubleshooting article. The error 21 is a hardware issue and is addressed in the quote from the second article.

iPhone: Hardware troubleshooting

http://support.apple.com/kb/TS2802

Resolve iOS update and restore errors

http://support.apple.com/kb/TS3694#10

Check for hardware issues

Related errors: 1, 3, 10, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 1000, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.

These errors mean that your device or computer may have a hardware issue that's preventing the update or restore from completing.

1. Check that your security software and settings aren't preventing your device from communicating with the Apple update server.
2. Then try to restore your iOS device two more times while connected with a cable, computer, and network you know are good.
3. Confirm that your security software and settings are allowing communication between your device and update servers.
4. If you still see the error message when you update or restore, contact Apple support.

Have a nice day,

Mario