Have you tried dragging your mouse along the bottom/sides of your screen, in case it is set to auto-hide?
If you still have your menu bar, go to
Applications > System Settings > Desktop > Dock to change its settings. If it isn't set to disappear, try toggling it hidden and unhidden to see if it comes back.
If it isn't back, let's check if it's running (the dock's process name is
plank) with a command that shows all running processes, filtered for all lines with "plank" (open Terminal from Applications):
ps cax | grep plank
plank isn't running, run the command
plank, and let us know what you see? You shouldn't have to install the dock (as in the other answer) because it comes with the operating system (unless for some reason it has been uninstalled).