Do you have just the one drive? A snipping tool picture attached of your disk management window might help.
Until then, try this. Open an administrative command window. type
Diskpart and hit enter.
Type the following commands and hit enter after each. You can also type just the first 3 letters of each one.
List Disk
This should show you your drive/drives
select disk 0
If you only have one drive, use 0. Otherwise use the number of the drive you want.
list partition
Post back what this result is, then select the partition you want and
select partition 3
this is the 3rd partition on the drive.
If you do not see the third partition, it does not bode well.
If you have the third partition, try this
assign letter=G
pick whatever letter you want, but the DVD has a letter, and so might any external devices.
Type
exit and
exit to close diskpart and close the window.
The help file says you cannot assign a drive letter to a partition that has a pagefile. Yours should not, but since you can't see it, who knows.
If none of this helps, you might download and burn the bootable
Partition Wizard and boot to it to check the drive.
All I might think is that something got messed up, as reghakr says.
There may be a way to rebuild the partition table, but this might be dangerous. and since I have had no experience, I can't really say. I have not used this software, but something like
this may be your only hope.