Missing parts when I build my game....

I set up a side-scroller/runner game and it works perfectly in the play panel of unity. But when I Build & Run my platforms and player disappear but my GUI is still there.

What is going on? (My game is based on the z-axis if that matters....)

EDIT: Comment and tell any other info you need to find out what is wrong.

Make sure you have selected the proper level(s) in your build settings. You might for instance at an earlier point in time made a test in a scene with the gui alone, and you might be repeatedly running that scene.

If you have tagged something "EditorOnly", then those objects are excluded from a build.

Check some of your objects if this tag is placed on them. If they do, remove the tag.