Hey Knead,
Issues with the editor are typically due to a Javasacript conflict caused by a theme or plugin conflict.
The latest versions of WordPress core (4.5+) use a newer version of the jQuery JS library that “fixes” a long-standing syntax bug. This bug previously allowed theme and plugin developers to use a technically incorrect selector syntax. Since the “fix” these themes and plugins have been encountering JS errors as a result.
If you’ve recently updated your WordPress to 4.5+ (as you should have), you should make sure that all of your plugins and your active theme are also updated to the most recent version in order to ensure compatibility.
If you’re still having this issue after updating all plugins and the active theme, you can narrow down the cause of the error like this:
1) Load the post edit screen
2) Open your browser’s developer console – F12 should do this in most browsers
3) Find the “Console” tab in the developer console
4) Reload the page
You should see some orange/red lines in the Console indicating a JS error and where it originated from. If you screenshot or copy/paste the details here, we might be able to help you discern next steps.
