The most often case while upgrading a website is that some of your content has shifted during upgrade. Some files may seem to be missing completely - do not worry! So long as you are running WordPress 4.1+ your mistakes are saved and can be restored from the WordPress editor's page you are looking to fix. Here's a few other things to go about restoring that website to "good as new".
1) Disable all plugins and re-enable all plugins ONE AT A TIME (refresh page in between)
2) Go to the Page Editor and restore the page to an earlier date (preview - not save - the page before updating)
3) Call us and we can help you fix it for a consulting fee of $50 / hr. Most troubles are solved in 1-3 hours.
NOTICE: This is for WordPress users only, although you can utilize the same technique in Joomla, Wix, and other CMSs.