On windows 8 IE 10 by default runs in METRO mode. But suddenly it stopped opening in metro mode. Then i checked what i have done why it is not launching in metro mode then i noticed and confirmed from various resources that only default browser can be run in metro mode. This was happening after installing Google Chrome browser on my Windows 8 and it was set as default browser. That is why IE 10 is not running in metro mode.
To fixing this issue i has just made IE 10 default browser again and it starts working normally.
To making IE 10 default browser goto IE 10->Tools->Internet Options->Programs tab.
Then on that tab you can see ?Make internet Explorer Default browser? link, click on that link it will open set default program control panel dialog with list of applications. Now select internet explorer in programs list and click on ?Set this program as default browser option?. This will make Internet Explorer as default program on Windows 8.
Now you can check by launching IE 10 from tiles program list.