I have a Dell Inspiron 15 with ATI graphics card. Everything was working without any problems until kernel 3.8. During the regular updates Ubuntu suggested to update to kernel 3.13.0.30. After that the operating system is not able to load in normal mode. Grub appears and I can login to failsafe mode with all kernels including 3.13.0.30. I can also login normally when using kernel 3.8. When using 3.13 normal login will endup to a blank screen before lightdm loads (before the login screen).
SnapOverflow
I found out it appears in Ubuntu 14.04 as well. The thing is that in case brightness adjustment does not work, adding the option "acpi_backlight=vendor" in the GRUB_CMDLINE_LINUX_DEFAULT variable in the /etc/default/grub file is a popular solution which now causes the booting problem. Removing the option from the file and updating grub by running "sudo update-grub" solves the problem.