IF the external drive is not showing up in File Manager, that implies a file system issue with it such that Windows is no longer assigning it a drive letter.
In the search area, enter "create and format" (without the quotes) and select the option that is listed. This will open the Disk Management console. Scroll down through the list of drives to see if your external drive is listed. If it is shown then use the option to assign a drive letter. If that works, you will be able to access the drive.
IF the drive entry say "raw". that means the file system has become trashed and you will have to use third-party commercial drive recovery software to see if you can get the file system restored. We have no way of assisting in that work.
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