Sometimes, Ubuntu freezes at Ubuntu loading page when it is booting to a log in page. The problem may be caused by updating some latest packages in the previous time you used your machine. There are many solutions posted on many pages. I'm going to tell you a usable solution for a newbie in Linux like me.
Firstly, I go to recovery mode of Ubuntu (there is no safe mode here.) I use the Ubuntu in a dual boot machine, Windows 7 and Ubuntu 12.04. To do so, I immediately press left Shift button after selecting an option of Ubuntu from the boot menu, which appears to have Windows 7 and Ubuntu.
Then, I choose a previous version of Linux kernel. The navigation leads me to several versions of Linux kernel. I choose the latest one which is not the current Linux kernel. After that, the system will reboot and navigate you to the previous version of Linux kernel without freezing. You may start looking for the root of problem here.
The reference is following link:
http://askubuntu.com/questions/135509/how-would-i-boot-into-the-ubuntu-and-windows-recovery-modes-if-i-removed-the-opt
Good luck,
Saran
