If your Ubuntu installation locks up as the graphical boot screen attempts to initialize, and you can't get access to your system after starting it up, booting into a previous version can be a help.
This can happen after updating to the the latest update to Linux 4.4.0-108 kernel, which included a fix for the Meltdown vulnerability.
You should be able get back into Ubuntu again by going back to the previous working kernel version from GRUB, in this case 4.4.0-104-generic:
- Reboot while holding down the Shift-key
- Select Advanced options (second option) and hit Enter
- Select 4.4.0-104-generic (third option) and hit Enter
The reason I include the position in the list is because my monitor has a huge resolution, making the text unreadable.
How to see your versions of CPU, Ubuntu and kernel:
$ cat /proc/cpuinfo
model name : Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
$ lsb_release -a
Description: Ubuntu 16.04.3 LTS
$ uname -a
Linux comp 4.4.0-104-generic #127-Ubuntu SMP Mon Dec 11 11:16:42 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
See also: https://ubuntuforums.org/showthread.php?t=2382157
EDIT: A new Ubuntu release has just been issued, so updating your system should result in kernel 4.4.0-109 getting installed, which seems to work for most people:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1742323
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1741934
