Fixing A Broken Drain??

How do fix a bathtub drain? The faucet works great, but when you lift the little drain plug switch, nothing happens. Our upstairs bathroom has 3 inches of water from yesterday's baths that I can't get to drain. I've looked it up online on a few sites, but I haven't been able to find anything that helps. I SOOOOOOO don't want to call a plumber, I can't afford that this week! I may have to just bite the bullet and unscrew the face plate to see if there is something I can figure out by looking at it. If not, it isn't a problem that can wait, so I will have to call someone.

Any ideas??