Why does this happen?
Sometimes we just get an incomplete update. So a quick fix is go to Updates then click the Re-install button
.htaccess
If this doesn’t work, delete or rename your .htaccess file, then go to Settings > Permalink and save changes, this will create an .htacess file for you.
CDN
You can also disable your CDN, sometimes JavaScripts does not load because of the CDN.
That’s it!