How do I reboot or restart a "frozen" or "hung" iPhone? How do I reboot or restart a "frozen" or "hung" app or otherwise "force quit" an app?

Apple explains how to turn off and reset the iPhone in the manual for each model (on page 52 for the iPhone 4 models, for example) as well as on the company support site. However, as this is a very frequently asked question, the answer also is provided below for your convenience.

Restarting iPhone

Apple notes that a restart -- turning it off and back on again -- should be the first effort to resolve a "frozen" or "hung" iPhone. The procedure is:

Press and hold the On/Off Sleep/Wake button until the red slider appears. Slide your finger across the slider to turn off iPhone. To turn iPhone back on, press and hold the On/Off Sleep/Wake button until the Apple logo appears.

Resetting iPhone

If turning the iPhone off and back on again doesn't fix the problem, or the iPhone is unresponsive when you try to turn it off, you have to reset it by doing the following:

Press and hold the On/Off Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears.

Force Quitting an App on an iPhone

If you have a single misbehaving app on your iPhone, you can "force quit" it by doing the following:

Press and hold the On/Off Sleep/Wake button for a few seconds until a red slider appears, then press and hold the Home button until the app quits.

If none of the above solves the problem, you may need to restore the iPhone to its original factory settings. Please note that restoring an iPhone deletes all of your data (songs, photos, videos, apps, and so on) on the iPhone. Be absolutely sure that you have backed up the latest version of the content on your iPhone before restoring it.

Apple's official instructions to restore an iPhone are:

1. Connect iPhone to your computer.

2. In iTunes, select iPhone in the Devices list, then click Summary at the top of the screen.

3. Click "Check for Update." iTunes tells you if there's a newer version of the iPhone software available.

4. Click Restore. Follow the onscreen instructions to complete the restore process. When restoring, it is recommended that you back up iPhone when prompted. When the iPhone software has been restored, you can either set it up as a new iPhone, or restore your music, videos, app data, and other content from a backup.

It is hoped that one of the above will help you solve the problem with your iPhone.

If after trying all of the above your iPhone still is misbehaving and it seems likely that you have a hardware problem, see "How do I send in my iPhone for service?"