Except for the stylings applied on the textarea, the theme doesn’t have any functions concerning the post editor. You probably have conflicting plugins.
Have you recently installed a plugin or updated anything?
- If the editor works when using a different browser then the browser cache is the culprit. Try clearing your browser cache.
- If you have a caching or minifying plugin, clear or refresh.
- Try deactivating plugins to check if one is causing the issue.