If you are in this situation, there are two options:
1. Hold down the home and power buttons until you see "Connect to iTunes", then use iTunes to restore to the latest firmware. This will lose data stored on the phone.
2. Apply a quasi-experimental fix using redsn0w. Download:
● redsn0w 0.9.6rc19: Windows / Mac (do not use an earlier version)
● Fix bundle, depending on firmware version: 4.2.6 / 4.2.7 / 4.2.8
● IPSW for your firmware version, if you don't have it: 4.2.6 / 4.2.7 / 4.2.8
To perform the fix:
Open redsn0w, select your IPSW, choose "Install custom bundle", and select the fix bundle. Then follow the directions. If it works correctly, after installation your phone will boot normally.
There is a separate bug where choosing "Re-jailbreak" would cause a device to be unable to boot. If you have this issue and don't want to restore, you can contact comex via the link below...