If your phone is unresponsive, it might display a big exclamation mark, a combination of a gear and lightning bolt, or it gets stuck showing your OEM's logo: 



If your screen displays any of these images, try this: Press and hold both the volume down button and the power key for 10 seconds. This should get your phone to restart properly again.

Software updates add new features that improve the performance and stability of your phone and may get it working again. If restarting your phone didn't resolve your problem, try updating to the latest version of Windows Phone 8. 

To update your phone, follow these steps:

1. Swipe left to the App list, tap Settings, then tap Phone update.
2. Tap Check for updates. 
   Note You may have to make some room on your phone for the update. If necessary, you may have to delete apps, music, videos, or other media first.
   

How to install the update

1. Tap Download when prompted.
2. Do one of the following:
   
   • Tap Install, tap Show details, and then tap Install again. Your phone will restart, and then install the update.
   • Tap Postpone if you want to install the update later. To restart the update, go to Settings 
     
     and then tap Phone.
   
   Note It typically takes 5-10 minutes to install an update, but may take longer.
3. After your phone restarts, wait for it to migrate your settings, and then tap Done to finish the update.

Resetting your phone will return it to the same state it was in the first time you turned it on. All your personal content--including text messages and apps you've downloaded--will be erased, and the phone will be reset to its factory settings. Here are two sets of steps to follow, depending on whether you can access your phone's Settings screen or not.

If you can access your phone's Settings screen:

Sync your data before the reset and restore. If you decide to reset your phone, you can back up apps, text messages, photos, and more to your Microsoft account in the cloud beforehand. For more information about backing up your phone, see Back up my stuff. 

To back up your phone's information to the cloud:

1. Swipe left to the App list , tap Settings 
   
   , and then tap Backup.
2. Tap App list + settings, and then tap Back up now.

To reset your phone:

1. Tap About, and then tap Reset your phone.
2. You'll receive two warnings. If you're absolutely sure you want to restore your phone to its factory settings, tap Yes, and then tap Yes again. (It might take a while for the process to complete.)

If you can't access your phone's Settings page:

1. Turn off your phone.
2. Turn on your phone again by pressing the volume down button until you see the exclamation mark 
   
   .
3. Then, reset the phone by pressing the following four buttons in this order: Volume up, Volume down, Power, Volume down.
4. Your phone should now reset and restart itself. (It might take a while for the process to complete.)

To restore your phone

Once your phone is reset, you will be asked to set up your phone again (as if you had just purchased it). In the "Let's get started" process, enter the Microsoft account you originally entered. This will let you select a previous restore point. (If you receive a list of restore points, select the most recent point.)

1. When you first set up your phone, sign in using the same Microsoft account you used when you created your backup.
2. When prompted to restore your phone, tap the backup you want to use, tap Next, and then tap Next again.
3. Wait for the backup to download, and then tap Next.
4. Do one of the following:
   
   1. Enter passwords for your restored accounts, and then tap Next.
   2. Tap Skip if you want to enter your passwords later.
5. Tap Done.
   
   http://support.microsoft.com/kb/2749480