Welcome to the community, @Ooohsehun!
Unfortunately, you could be dealing with a failed external HDD just like @CountMike mentioned.
I'd advise you to check how the HDD will get recognized in another computer. If it still struggles getting detected, then indeed, your HDD has most probably failed. However, you should check the warranty on the external drive from the manufacturer's official website. If it's still covered, contact them and send an RMA request to get a replacement. If is not under warranty, you could also try taking it out of the enclosure and plugging it via SATA internally to your computer.
If you have any important data on it, that you wish to retrieve. You should better contact a professional data recovery company for assistance. They are your best bet on getting any of those files back.
Keep us posted with the troubleshooting, though. Good luck!
SuperSoph_WD