Connect the iOS device to your computer and restore via iTunes. Place the iPod in Recovery Mode if necessary to allow the restore.
For how to restore:
If You Are Locked Out Or Have Forgotten Your Passcode
If you have forgotten your Restrictions code, then follow the instructions
below but DO NOT restore any previous backup. If you do then you will
simply be restoring the old Restrictions code you have forgotten.
A Complete Guide to Recover Your iDevice if You Forget Your Passcode
If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup). If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present. Refer to Updating and restoring iPhone, iPad and iPod touch software.
Try restoring the iOS device if backing up and erasing all content and settings doesn't resolve the issue. Using iTunes to restore iOS devices is part of standard isolation troubleshooting. Restoring your device will delete all data and content, including songs, videos, contacts, photos, and calendar information, and will restore all settings to their factory condition.
Before restoring your iOS device, Apple recommends that you either sync with iTunes to transfer any purchases you have made, or back up new data (data acquired after your last sync). If you have movie rentals on the device, see iTunes Store movie rental usage rights in the United States before restoring.
Follow these steps to restore your device:
1. Verify that you are using the latest version of iTunes before attempting to update.
2. Connect your device to your computer.
3. Select your iPhone, iPad, or iPod touch when it appears in iTunes under Devices.
4. Select the Summary tab.
5. Select the Restore option.
6. When prompted to back up your settings before restoring, select the Back Up
option (see in the image below). If you have just backed up the device, it is not
necessary to create another.
7. Select the Restore option when iTunes prompts you (as long as you've backed up,
you should not have to worry about restoring your iOS device).
8. When the restore process has completed, the device restarts and displays the Apple
logo while starting up:
After a restore, the iOS device displays the "Connect to iTunes" screen. For updating
to iOS 5 or later, follow the steps in the iOS Setup Assistant. For earlier versions of
iOS, keep your device connected until the "Connect to iTunes" screen goes away or
you see "iPhone is activated."
9. The final step is to restore your device from a previous backup.
If you can not restore your device then you will need to go to recovery mode.
Placing your device into recovery mode:
Follow these steps to place your iOS device into recovery mode. If your iOS device is already in recovery mode, you can proceed immediately to step 6.
1. Disconnect the USB cable from the iPhone, iPad, or iPod touch, but leave the other end
of the cable connected to your computer's USB port.
2. Turn off the device: Press and hold the Sleep/Wake button for a few seconds until the
red slider appears, then slide the slider. Wait for the device to turn off.
If you cannot turn off the device using the slider, press and hold the Sleep/Wake
and Home buttons at the same time. When the device turns off, release the Sleep/Wake
and Home buttons.
3. While pressing and holding the Home button, reconnect the USB cable to the device.
The device should turn on. Note: If you see the screen pictured below, let the device
charge for at least ten minutes to ensure that the battery has some charge, and then
start with step 2 again.
4. Continue holding the Home button until you see the "Connect to iTunes" screen.
When this screen appears you can release the Home button.
5. If necessary, open iTunes. You should see the following "recovery mode" alert:
6. Use iTunes to restore the device.
If you don't see the "Connect to iTunes" screen, try these steps again. If you see the "Connect to iTunes" screen but the device does not appear in iTunes, see this article and its related links.
Note: When using recovery mode, you can only restore the device. All user content on the device will be erased, but if you had previously synced with iTunes on this computer, you can restore from a previous backup. See this article for more information.