In Revit you can hide elements and categories within a view. Turning categories back on is easy through the Visibility Graphics Overrides, but turning elements back on isn't...
...or is it?
In this example the roof is hidden, the Visibility Graphics Overrides won't help us as this works on categories which are already turned on.
So we need a way to be able to select the hidden roof and manually unhide it.
This is where the reveal hidden elements tool comes in.
At the bottom of your workspace, you have a row of graphic tools, in here you can find Reveal Hidden Elements (It looks like a light bulb)
Clicking this will in view, reveal all hidden elements and colour them in purple. You can now select your elements, right-click it and unhide.
Clicking the Reveal Hidden Elements button again will restore your view back to normal.