Ubuntu 14.04 suddenly won't boot? This is how you debug it!

After a couple days without rebooting my X201 that was running Ubuntu, I decided to reboot the system. And unfortunately, it won't boot afterwards! After passing the Ubuntu's familiar purplish-black blank background, it got stuck in a black screen. What scared me were that pressing caps-lock didn't even turn on the keyboard indicator and I also couldn't enter "console mode" (ctrl + alt + f1). I was scared that the system went into a hard-hang, perhaps because I accidentally installed some craps. However, it turned out that my xserver binary was missing; the file /usr/bin/X didn't exist, for some reason! How did I figure that out? This post explains it.

Ubuntu keeps different the logs of the last boot attempt. These logs can be used to triage booting issue and potentially are useful to fixing it too. It Through these logs, I was able to figure out my xserver issue. This is how you do it:
1. Boot the system, get it to the hanging state. This step is necessary in order to get the log files generated.
2. Boot to a live-session using Ubuntu CD or Ubuntu USB stick
3. Open "[path to partition]/var/log" of the partition where the problematic Ubuntu is installed. Note that this is not just "/var/log" because that belongs to the currently running live session.
4. Sort the log files by modified date, and open the logs that correspond to the booting attempt from step 1.
5. As there are different log files, I'd suggest to first look at boot.log, which is similar to the kernel log that you get from dmesg. If nothing is suspicious there, then it's likely to be a userspace issue.
6. In my case, I found my xserver problem through the log at /var/log/lightdm/lightdm.log. This is the suspicious snippet of the logs:
+0.05s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log
[+0.05s] DEBUG: DisplayServer x-0: Can't launch X server X -core, not found in path
[+0.05s] DEBUG: DisplayServer x-0: X server stopped

Through a quick google search, I figured that /usr/bin/X is where "X server X -core" is supposed to be located at. In my case, the file was missing, so it was rather obvious. I then rebooted to Ubuntu recovery mode (by holding "shift" right after the system passes bios), and reinstalled xserver via sudo apt-get install --reinstall xserver-xorg.

Hope this post can be useful!