Hi Trudi,
I’m just a regular user like you but I had the same problem when I installed 3.9. What fixed it for me was to deactivate all my plugins, then reactivate one, try the visual editor in posts and pages, and if it worked OK, then reactivate another one. I kept doing it till I found the plugin that wasn’t compatible with 3.9 (caused the Visual editor problem again after I activated it). For me that was the Zedity plugin, but within a few hours they sent an update – some other plugin developers might not be so quick though!
Good luck!