Even if Windows Phone 7.5 is a stable operating system, sometimes you can experience OS crash or OS freeze. We all know that Lumia 800 and Lumia 900 have battery that can not be removed, in order to reset it you must use a combination of keys.

How to Soft Reset Lumia 800

In order to reset your Lumia 800, you must press the volume up, volume down and power keys for about 5 seconds. After that, phone will reset.

How to Soft Reset Lumia 900

If you want to reset your Lumia 900, you must press the following keys for 5 seconds: volume up, volume down, camera and power.

If this fails, both Nokia phones have another method: press power button for about 10 seconds.