The first step to take when any separate hard drive internal or external isn't seen with an assigned drive letter is to vist the Disk Management tool. That's found in the "Control Panel>Administrative Tools>Computer Management>Storage".
If the drive is seen "offline" that will mean you will need to see it initialized provided there is an MS partition on the drive like Fat32 or NTFS and not raw, unpartitioned or any other partition type for another OS like Linux. For Linux you would need a separate tool available to see it mounted in Windows.
Sometimes all you need to do there is click the refresh or rescan options if there isn't any prompting to format the drive. Then you simply right click on the visible partition and select the change drive letter option in order to select a free drive letter.