I was in NuGet and I was looking at the available Updates.
I saw there were updates available for MVC, Entity Framework, jQuery and others so I updated them.
After I did the update, the "Add to Cart" button didn't work.
Also, when I hit F5 to run, I get about 5 "unhandled exceptions" errors in jQuery files like "jquery.validate.min.js".
The original version of jQuery on Nop 3.3 was : jquery-1.10.2.min.js
The new version is : jquery-2.1.0.min.js
I deleted everything in Scripts folder and replaced them back to the original 3.3 contents and its working again.
So be careful updating jQuery.