Can you see the disk in Disk Manager?
To run disk manager: Start -> Right-click "Computer" -> Manage -> Click the "Disk Management" link in the left pane.
Look for your disk in the lower portion of the right pane. If it's there but doesn't show a drive letter, then right-click it to assign a drive letter.
If the disk isn't there, click on "Device Manager" in the left pane and look for icons with yellow exclamation marks - they indicate hardware that's not working properly.