Start by using a browser like Firefox with Firebug (my pref) or using Fiddler2 so you can at least make sure what files and content is getting into your browser. (the Net tab on Firebug will show you if files are being loaded).
Then try finding various piece of the javascript in Firebug (or other debugger) and put some breakpoints in and see what's going on, whether your code is even being executed. Look in the Console tab for errors.

Some of these are typical web programmer tools and tasks, so if you don't know what I'm talking about you might need to start there -- Google "Firebug" and find a tutorial and get used to them. If you don't, you'll have a harder time getting to the bottom of this.

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Also make sure that Modernizr is being loaded as early as possible, prior to any other scripts. If you are using the Client Resource Management DnnJsInclude, use the PageHeaderProvider to make sure it goes in the head, and set the Priority lower than jQuery (5), like this: