This sounds like its a new card and if thats the case you would be better off calling the tech support from the company that built it.

However try this before you sit on the phone, >> START >> RIGHT CLICK "My Computer" >> CLICK "Manage" >> CLICK "Disk Management" >> find the drive in the bottom window pane on the right >> RIGHT CLICK THE DRIVE "Chage Drive Letter" >> Try to add a drive letter to it.

If that was confusing I made a quick video showing you how to do that.

http://www.zigzap.net/driveletter.avi