i had the same problem
this is how i fixed it
Log into your WordPress admin
Go to Design > Themes
Switch / activate any other theme than your current one (I just use the Default)
Go to Write > Post (as if you are about to write a new post)
You should see the GUI for the editor restored and be able to switch between Visual and HTML modes once more
Go back to Design > Themes and reactivate your desired theme
If that doesn’t work … I have NFI.