This questions is closely related with previous questions asked here. But there are some differences. I was trying to upgrade from 12.04 to 14.04LS. While download the packages, the system hanged suddenly. After waiting for an hour, I had to forcefully close everything. After this, the following things happened.
1> The last Linux kernel is not working and has been spoiled.
2> I used an older version of the kernel and managed to get internet connection. After this, I can not log in. Indeed I am getting the login screen. It does not allow allow me to log in and when I try, I get a similar screen like this and then it log out.
I believe that the unity is broken. I could manage to upgrade by using the command prompt. Still the problem is not getting sorted out. In fact, I tried the suggestions of the questions asked above. Using it, I can manage to get a graphical prompt for the guest session, not for my user id. Please help.