It happened to me before and it happened again, recently. And, as always, I forgot the solution. And equally as always, Google was my friend. Google, and some unknown WP user with a better memory than mine!
So, the solution is to click on the ‘User’ tab on the left in your Admin panel, then make sure the ‘Disable Visual Editor’ option is NOT checked!!
Yes. Duh !
For some reason it got changed on 2 of my many blogs and not on the others. It may have to do with reloading the blogs from a backup.