Let me guess: a Dell PC/Laptop? Dell installs Intel Rapid Storage Technology software and drivers when it really isn't necessary. With it installed, it is very easy to click on the wrong thing and turn your D drive into a (RAID 1) backup for the C drive.
If this is the case - which I am pretty sure it is - you have had everything on your D drive completely wiped out. I hope you have an offline backup of its contents because that is the only way you will be getting it back. Do make the drive reappear, you need to remove the two drives from the RAID array. Your boot drive should still be ok (touch wood).
Sorry, reality bites,
...S
