Wierd. Did you try unplugging it from system and replugging it back in? Is it an IDE style or SATA drive. How old is it, who makes it?
Is it the only HD you run in the machine? Does the PC boot up okay? If the PC boots and everything runs, but for some reason you can't see it in certain places then I am unsure of what that could be. I'd try replugging it in first to see if maybe the connection just needs to be re-recognized by the software/hardware. If that doesn't work...I dunno, I'm gonna search this one though.
AMD 3400+ 2.4GHz
1GB DDR Crucial RAM