What did I do? I somehow changed my view setting and I can no longer select anything with RMB. Please see screenshot:
You have ticked an option that displays only objects that will be rendered. Open the Properties panel with N and untick Only Render under Display. You can actually select objects (the 3d manipulator widget is still being shown) and move them as normal, you just cannot see the visible outline.
Came here because my grid floor plane had disappeared as well. Realized it was because the 3D viewport shading was set to "Render". Changing it back to Solid fixed it. Shift+Z