Javascript & Ajax failed to load on many pages like field editing & views editing pages. The site I'm managing is runs by Drupal 7.53 with many contributed modules. 

I don't know what happened from yesterday, ajax queries in field editing pages and views editing pages throwing errors or not loading at all. Clearly I can see what is causing this error in JQuery and Ajax. 
All the modules installed are contributed modules except one custom module, which doesn't have any Javascript code in it. Cleared the cache few times . No javascript and CSS aggregation. But still wherever JQuery &amp; Ajax is needed , the pages are breaking . How can I solve this problem. How can I figure out what is causing problem? 

I'm unable to alter any of the views due to ajax error throwing up. 
All help is appreciated.
Jquery Update module is very helpful but new jquery versions can cause issues with other dependent modules. The module is important not only to keep your site up to date but also allow for theme-specific control to ensure compatibility. So the module's settings page is something you might want to visit and test when a new jquery version is released.

Also, the CDN option can cause load issues if the provider you choose is offline for some reason or overloaded. From the jquery update settings warning:


  "NOTE: Using one of the "CDN Provider" options below is the preferred
  method for loading Bootstrap CSS and JS on simpler sites that do not
  use a site-wide CDN. Using a "CDN Provider" for loading Bootstrap,
  however, does mean that it depends on a third-party service."


So use this option with caution.