Dock is not showing up on my iMac
Apparently you have accidentally set the Dock to automatically hide by invoking the keyboard shortcut Command + Option + D. Re-invoke the same keyboard shortcut to toggle the setting and turn off auto hide.

You can also effect the same change by going to Apple Menu → System Preferences → Dock and uncheck Automatically hide and show the Dock setting.



If you wish to keep the Dock hidden, the same can also be shown as desired by moving the cursor to the edge of screen (by default Bottom) where the Dock is positioned (as seen in the Position on screen setting in the screenshot above).