A student of mine was working on a Snap! project when for some reason the variables section just went black. She can still find and use those blocks by searching for them from another window but that particular window is just black. We tried:
Reloading the page
Turning the computer off and then on again
Exporting the project and then re-importing it to another user's Snap! account
Welcome to the forums!
I think I know the problem, this happened to me too in the past:
Sometimes the category disappears when you use an apostrophe in a custom block. This is due to the fact that Snap! uses apostrophes internally to compile blocks and it messes up the rendering.
The only solution I know of is to export the XML, remove the apostrophe, and import it back to Snap!.