It's possible that the drive has been disconnected (virtually) from Explorer. If you are using Windows XP or above, go to the Control Panel > Administrative Tools > Computer Management then in the new window click on Disk Management. In the bottom half of the window you should see a drive that has all three partitions side by side with their corresponding drive letters. If the one that is supposed to be V: isn't labeled as such, right click and use "Change Drive Letters and Paths." If it doesn't appear whatsoever - if it appears as only two partitions for example - then something might have happened to the drive.

Hope this helps.

If the drive has the assigned letter "V" and you insert a USB flash drive that also has the same letter assigned, on startup you you may find that the flash drive has grabbed the "V" letter and then the "V" partition will not be seen.
Go into Computer management in Administrative tools Disk Management and look at the drives there and if you see the letter "V" is being used by other than the partition, assign another letter to that drive.
Close Administrative tools and open My computer and you should see the missing partition.
If that doesn't work, boot from and use Partition Magic to examine the total drive.
You may need to use Norton Disk doctor to repair the partition.
As a last resort, save your data to an external drive and rebuild the computer at which time you will be able to repartition the drive.