How to Restore Your iPhone After Jailbreaking Failsby Andrew Smith
Jailbreaking an iPhone gives you the ability to add non-Apple applications and functions, like live FM radio or PDF readers, onto the phone. The jailbreaking process is not always a successful one, however. You may be forced to restore the phone back to the original Apple settings after a jailbreaking attempt has failed. If the phone is not functioning properly, though, you will need to place the device into a special mode before restoring it.
Charge the iPhone for 30 minutes. A computer or iPhone charger can be used to charge the device.
Turn the iPhone off or hold down "Home" and "Power" on the phone until it powers down.
Connect an iPhone/iPod USB cable to a computer that has iTunes installed on it. Do not connect to the phone yet.
Open iTunes on the computer.
Hold down the phone's "Home" button. Then, connect the iPhone to the iPhone/iPod USB cable. Keep holding "Home" until a "Connect to iTunes" screen appears on the phone. A dialog box opens inside of iTunes. Click "OK" in this box.
Click the iPhone menu in the iTunes Source Pane, which is under "Devices." Then, click the "Summary" tab in iTunes.
Click "Restore" inside iTunes. A small box appears. Click "Restore" inside this box. The iTunes Software goes into "Restore" mode. Follow the instructions inside iTunes to restore your iPhone to your preferences (via a previous iPhone backup, with or without a software update, etc.).