Where C: went
Type this command in a terminal window
this will list all the mounted partitions and their stats in human readable format.
Look for any partitions either by size or by the "Mounted on" column that matches your "C: drive"
Take note of the size and partition number of the "D: Drive" that you can not access in windows
sda = 1st hardrive
sdb = 2nd hardrive
sda1 = 1st partition on 1st harddrive
sda2 = 2nd partition on 1st harddrive
Where D: went
In Windows click Start -> then right click on Computer -> click on Manage -> Disk Management
Here you should see any detected drives you have on your system and the partitions that are on them. You will want to leave any partition that say unknown alone as this is where Ubuntu lies. Look for any NTFS partitions that are not labeled (C:), this is most likely you missing drive. Right click on the NTFS partition and choose "Mark Partition as active". Read any warning that may appear (just in case). The drive should be given a drive letter and should then show up under Computer as expected.