Boot stuck at "A start job is running for Create Volatile Files and Directories"

This message will appear when the /tmp directory is full or has too many files/directories inside it. Since it prevents a successful boot, we will need to modify the boot options to fix it

First, restart the affected system (Ctrl-Alt-Del or power cycle it) and upon reaching Grub, press the E key to begin editing the boot option

Scroll to the bottom and find the section including root=/* and "ro". This is the area we will be editing. Server versions of Ubuntu normally mention "maybe-ubiquity" after this, whereas desktop versions normally include "quiet splash"

Change the "ro" (read-only) to "rw" (read-write) and add init=/bin/bash after it (ensuring there's a double space after rw)

Now we're effectively running in single-user (root) mode. We can now run the commands needed to reset the /tmp directory:

Move the existing directory:

mv /tmp /old.tmp

Make a new directory:

mkdir /tmp

Set the correct permissions:

chmod 1777 /tmp

Now we can reboot. The normal "reboot" command doesn't work in this mode, so a Ctrl-Alt-Del is the easiest way!