What you want to do is install Firebug, and examine the DOM element in that. It will show you any customized CSS properties, and you can check/uncheck them one by one. You will likely find the one that's causing the problem quickly.
If it's not actually disabled, but you can't interact with it, you can also see that in Firebug. Just use the inspect by mouse click feature, and it will show you which DOM element is on top of it, and what it's dimensions are.
