Visual Plug-in Culprits
From what I have seen from members here and reading the Wordpress forums, there are some common plug in culprits that can suddenly cause your visual editor to disappear. The main main culprits are plugins that add extra utility to the visual editor (probably why when they start acting goofy, goofy things happen).
First one is TinyMCE advanced. I wasn't aware that people were still using this because I know a while back it wasn't compatible with a big Wordpress update but apparently it has either been updated or people are just running an outdated version.
The second one is WP-edit. I know quite a few members here use this plug in currently and have never had problems while others have had their editors disappear suddenly without any warning.
Another common one is CKEditor but I actually do not know how current this is anymore. But a member did say that it blew up her site one time so you might want to reconsider it if you are using it.
In either case, if you have one or both of these plugins going. Try deactivating them and see if your visual editor reappears. In case you aren't familiar with deactivating plugins, just go back into your plugins area to see everything you have installed.
I know these aren't the plugins I am talking about but they will appear the same way. You will need to click the deactivate button on them. If the activate option is already available then that means that plugin is currently not active and should not be affecting your site in any way.
Tip for Still Using Visual Plugins
Thanks to Loes for mentioning in the comments what she does to protect her site. She has a test site that has TinyMCE Advanced installed on it. She writes all of her posts on that site then goes into the TEXT editor and copies that html code from there.
You would then take this copied code and go to your main site, create a post/page and paste that information into the TEXT editor. That will ensure that all the customization you made to your text sticks. So if you realize that maybe one of the above plugins is causing problems on your main site, then protect your main site and maybe do it this way.
Other Plug-Ins
Technically any plugin can start causing errors with your site, especially if you have recently added one or updated one.
Try disabling all your plugins and if your editor reappears then reactive one by one until you find the culprit.
Did deactivating a different plugin fix your problem? Please let me know below.
Still missing your visual editor?? More to go!