I had this same problem.
I wanted some more space for music that I could listen to from my Windows 7 system. So I shrunk my Mac partition and created a new "MEDIA" partition, with the intent of formatting it from Windows or something.
On restarting, I found I couldn't boot into Windows. (It didn't appear when I held down the Alt key. When I tried using System Preferences --> Startup Disk to boot into it, I was able to select the Win7 system, but on startup it gave me a black screen reading "No bootable disk". I deleted the MEDIA partition. No luck.
The Windows 7 partition is clearly intact. I can view it from OS X, copy files from it, and so on. It's right there in Disk Utility.
I'd really rather not reinstall everything and fiddle with all the settings all over again.
My guess: the ... boot sector? ... or whatever got messed up. Can I rebuild that somehow? Windows 7 is clearly still there.