I've written a long essay and was previously able to edit it. Now, though, I can open the editing tab from the left but the block doesn't have an 'edit' button. Have I somehow locked it inadvertently?

The only thing I can think of that I've done today was add some functionality to the editor toolbar via import, and possibly had the same page open for editing in two windows simultaneously.

Thanks!

Can't edit any WYSIWYG blocks, actually

Weirdly, it seems to be that the buttons for 'Edit' are in white i.e. invisible until hover?!

 

You would need to check this with this author of your theme, in order to get this corrected for you.