My volume and brightness keys aren't working due to change Fn settings?

Today, I was watching a youtube video with my niece on my laptop when she started pressing a bunch of keys (she's only 2) and brought up the settings on my computer. I saw a setting in the computer somewhere that had something to do with the Fn button on my keyboard, and I thought that she had changed it by accident so I switched it to the other one. Turns out, it didn't change, and now when I press the volume or brightness keys (which share a key with f keys- which are normally triggered by pressing the fn key) nothing happens. I have a Inspiron 1545 and I use windows 7, does anyone have any idea where in the settings I could change it back? I can't find it.