Issue Description
Some elements in your model adopted a transparent look in 3D, or only their contours are visible.
Solution
Layers: Status set to Wireframe
1: Go to the Layer dialogue box Document > Layers > Layer Options.
2: Check what Layer the wireframe/transparent elements are in.
3: Find the elements' Layer in your Layers list.
1: On the right of the Layer's name, you will find a render icon
2: There are 2 settings: a solid icon and a wireframe icon.
3: Click once on the render icon to change the status from Wireframe to Solid.
Design options: Fade Environment is Active
When working with Design Options it is possible to activate a setting that changes the appearance of elements to help distinguish if you are working on an Option or the Main Model.
To deactivate it:
1: Go to Window > Palettes > Design Options.
2: Uncheck Fade Environment in the Design Options palette.
TIP: Once you Accept & Merge a Design Option into the Main Model, the 3D display of the elements will go back to normal.