Windows 7 Ultimate x64 Custom-built desktop

I have a new desktop that I built a few months ago that has a four-year-old WD hard drive and a two-year-old drive. I had set it up so that the newer drive had Windows and most of my files on it while the older drive had my music library, some movies and games, and a backup of all of my documents. About a month ago, I installed some new case fans and, in the process, I temporarily unplugged my hard drive (while the computer was off of course - I took all the necessary precautions) for wire management. I plugged it back in, and didn't really think anything of it. At around that time, however, I noticed that my older hard drive wasn't showing up in Windows Explorer anymore but I didn't really have time to check into it (I had just started college) and I'm finally getting a chance to now.

That drive doesn't show up in Windows Explorer at all but it does show up in Disk Management. That screen shows the following: http://puu.sh/17mMN

Any idea what happened? Is there any way to recover my files?

Thanks in advance for your help!

EDIT: The music and games and stuff used to be on "Disc 1", the 465.71 GB of what is now showing as unallocated space.