Welcome to the TH community, Ganjoni!
I'd recommend you check how the external HDD is recognized and if it's listed as healthy with the appropriate file system in
Disk Management in Windows. Sharing a screenshot might actually be really helpful!
If everything seems normal with the external drive in Disk Management, I'd suggest you right-click on it and
try to change the drive letter assignment. This should make your drive accessible through the File/Windows explorer again.
Always make sure you backup your data to at least one more location (storage device).
Having only one copy of your files is not a backup solution!
Keep me posted if you encounter any further issues with it.
Good luck!
SuperSoph_WD