Editing Page in Dashboard

Try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, try switching to the Twenty Fourteen theme to rule-out a theme-specific issue (theme functions can interfere like plugins).

Ok, try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.

Are you using any sort of caching plugin, or do you see a “Clear/Purge Cache” button in your Dashboard or admin bar? If so, would you please try clearing the cache?

Is the whole screen blank, or just the editor content?

Oh, this is in the Theme file editor, not editing a page or post? If so, that’s a bit different from what the original user is discussing. Would you please open your own thread? https://wordpress.org/support/forum/how-to-and-troubleshooting#postform

Excellent, thanks for sharing your solution!