Use a DOM inspector (like the one built in to Chrome) to first determine if it's set to hidden because of a CSS rule or because it was programmatically set to hidden with a direct inline style setting.
If it was programmatically set to hidden, then the only thing I know of is to search all the javascript code for anything that could be changing the visibility.
Then, when you find some candidate lines of code, use a javascript debugger (my favorite is the one built into Chrome) and set breakpoints on each of those lines. Then, reload the page after the breakpoints are set and when one of those breakpoints is hit, you can step through that part of the code and see who is doing it. At that point, you can even look at the call stack and see what code called this and so on. Or, you can step through and out of this code into the higher levels that called it and see why they are doing it.
If it is CSS rules that are making it hidden, then you need to look at what triggers those CSS rules (classes, IDs, etc...) and figure out how to change one or the other so that your desired object isn't hidden by that CSS rule. Remember that classes can be added/removed from objects in the DOM via javascript so that could also be part of the cause.