Very first thing to try is to reload the latest savefile. Sometimes things can fail to load for some reason and just reloading will make it show up. Happened to me once for a mannequin, and even once also for a resource patch in an outpost! Both times just reloading fixed it. Might not work, but easy enough to be worth a try.

If reloading didn't fix it, we will need to use console commands. So be sure to install one of the achievement enabler mod in order not to lock yourself out of the achievements! (I use BAKA ACHIEVEMENT ENABLER SFSE[www.nexusmods.com])

And just to be safe, make a hard save before starting the recovery process!

First step is to find the reference ID of the missing object. Usually the way to do that is to have the item on screen, open the console, and click on the item. Of course this won't work since the object has disappered! (DOH!) But there are two workarounds:

1. Load a previous save file where the object has not disappeared yet, find out its reference ID and scribble it down on a piece of paper.
   
2. Or alternatively, open the console and type "save funclist 1". This will create a text file with every refID in the loaded save file. In this file you can search for the refID of the missing object.

Typically, objects such as mannequins and turrets are actually NPCs. And their refID will look something like "FF0320D2" for example.

Now that you found the refID of the missing object, simply teleport it back into existance at your player's position. This is done by entering two commands:

1. Type "prid [refID]" with the refID that you wrote down, without the square brackets and without the quotes.
   
2. Type "moveto player"

Voila! Hopefully the missing object has been teleported back to you. And you can now go into build mode to reposition the object at its rightful place!