Ubuntu 12.04 stuck at boot splash screen
Solution 1:
first you should check if you can change to the console output by pressing F2 or ALT-F2 (never remember which one )
If that does not work or do not show you some error you should reboot and enter the grub boot menu with pressing "Shift" while booting.
When you are in the menu you should remove the "splash" and the "quiet" entries from the kernel commandline.
This way you will get the "normal" output Ubuntu (and a lot of other distributions these days) usually hides from you.
With the error(s) you might see there you can debug your issue and fix it.