Why does the middle button not work?

The circular, center button on my iPod Nano 3rd Generation does not work. The buttons surrounding it and the touch sensitive wheel work fine.

I can't get the case to open to check it out. I did find a repair tool online to crack the case but I am on a tight budget.

With some force powered by frustration and a large screwdriver I can sometimes get the middle button to work. (I have to exert a ton of force for this to happen)

The button will push in and make a soft click; this is exactly how it was when I bought it. Problem is I can't click songs or go through menus because something is wrong. What's wrong and how can I fix it?