Recently I faced this problem and guess what! I found out a solution to this problem. You get that problem as there is/are some unmet dependenc(y)ies with ur installed package(s).
Once you see the loading page of Ubuntu while booting, press <CTRL> + <ALT> + <F1> . This will take you to the command prompt. Then, sign in by typing your user name and then the password.
After that, use the command
to get the login page or sometimes, you won't even get that. You will directly be taken to the desktop. The problem is solved now.
Hope that helps !