Why don't my views show up in Storyboards and are greyed out in the left pane?

Question

I am experiencing a weird bug in Storyboards.

I set up a UITable

Answer 1

This happened to me because I copy and pasted some of my views from nib into storybaord. Turning size classes off, and turning it back on fixed the issue. This can be done in File inspector, using a checkbox named Use Size Classes

Answer 2

Just found an answer myself, it was because Size Classes were reset after I moved the project...

Greying out views is Xcode's standard behaviour when the views are only set for a specific size class which is currently not selected!

I had set the size classes for my UIViewController to Compact - Any, this is what it looks like in proper state:

The problem occurs when I change the size classes to another one (e.g. Any - Any):