I have been placing static farmhouses in my world, using the render window but suddenly they disappear. They are still there, and still clickable, but do not show. Is there a way to make them show without having to load another world space, then loading my own? I need to see them to lay out my town.

Try to refresh your Render window using F5. Objects often disappear for me too but sometimes a cell needs to be reloaded to show all objects again.

Niborius is correct. In exterior spaces if you move the camera around and cause it to load new cells several times objects can become invisible. F5 forces a render window refresh and should fix the issue.