Dashboard layout is crashed

Try switching to the Twenty Fourteen theme to rule-out a theme-specific issue (theme functions can interfere like plugins).

If that doesn’t work, 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.

Not quite what I suggested, but you should probably go back to 4.0 at some point, unless you like publicly known security vulnerabilities.