If that is your CD-ROM/DVD drive like in most windows systems now a days then the solution could be as simple as putting a disk in the drive. I know on my Windows 7 desktop system when there is no disk in the drive it is not visible on Explorer. If it is not your CD-ROM then I would go into Control Panel, Device Manager and look under Drives to see what device drivers are loaded. In Disk Management did you look at the bottow to see if the drive was there or only at the top listing?
If the drive is on the same disk then when you go into Administrative Tools, Computer Management, then Disk Management. In the center frame you will see the Volume List at the top and a Graphical Representation of the drives at the bottom. If you look at the drive in question in the bottom of the center frame you will be able to see any partitions that exist on the drive. If the partition has a drive letter assigned it should show here and it will also show if the partition is formatted and the format used something like mine are shown in the attached image.
If the partition still exists but simply does not have a drive letter assigned you can assign one by right clicking on the partition and selecting Assign Drive Letter or Path. Let us know what you find if you are still having problems after this.
The next question then is which version of Windows are you running? I am running Windows 7 and if you notice my primary HD has a System Reserved partition and it does not have a drive letter assigned. This is normal for Windows 7 as it boots from the System Reserved partition and switches the second partition to have the drive letter C: during the boot process.
It may have been helpful to specity if you "D" drive is removable such as a NAS for example. If that is the case here, please specify the unc with \ -\ and not // to see and access the drive. This can be especially true in the current windows version.
You indicated that the drive was previously visible, so try this and let me know.