Welcome to the TH community, smn468!
You might want to check how the external HDD is recognized in Disk Management on your computer. If you can see it there, I'd recommend you to try
changing the drive letter & paths of the HDD.
This could help you gain access to the drive on your computer again. Once you are able to do that, you should consider backing up all your data. Always have a backup of the files stored on a different location, this is the best way to avoid data loss.
Hope this helps. Keep me posted with the troubleshooting!
SuperSoph_WD