It is possible that another device grabbed the drive letter that was assigned to your external drive. The links below will help you resign a drive letter if needed.
How to change your drive letter in Windows Vista and Windows 7
How to change your drive letter in Windows XP
If the drive is not showing up in Disk Management then this is a sign that there is a problem with the drive. I would recommend connecting it to another computer to see if it will detect. If it does not then there is a problem with the drive and I would contact the manufacture of the drive do see if you are still in warranty.
Let me know if you have any more problems or quesitons
Answers & Comments
Jfmartinezmx
It is possible that another device grabbed the drive letter that was assigned to your external drive. The links below will help you resign a drive letter if needed.
How to change your drive letter in Windows Vista and Windows 7
How to change your drive letter in Windows XP
If the drive is not showing up in Disk Management then this is a sign that there is a problem with the drive. I would recommend connecting it to another computer to see if it will detect. If it does not then there is a problem with the drive and I would contact the manufacture of the drive do see if you are still in warranty.
Let me know if you have any more problems or quesitons
Issken
http://www.fixya.com/users/issken