Today I came from the office at 2 o’clock. After eating little bit, I started my laptop and it stuck at one point. I turned it off and started it again but no luck. I tried to login as another user but no luck. I was completely stuck. I was in tension what happened to my laptop because my whole project was in laptop and I am about to graduate within few days. (Thank god, I am taking backup of my project everyday. I save the backup in laptop as well as my email account.). I started laptop in safe mode. (You can start the computer in safe mode by pressing F8 while it boots). It was working fine so I took deep breath. I am using my laptop at office. I think the problem occurred because I installed one program from my office login. I restored the system and now it is working fine.
PS: So the moral of the story is
1) Take backup regularly. Never be lazy.
2) Computer is like life(not wife). Both can betray you anytime.