fn key not working

I have a late 2007 MBP, running 10.5.8. I hardly ever need the fn key, but I noticed today that it doesn't work. I wanted to hit F5 to refresh a page, but it just increased the volume instead. I went to system preferences and changed F-keys option, and then I could hit F5, but holding down the fn key still didn't work to get the special functions. How could one key on my keyboard be broken? Could it be a software problem?