Welcome to the community, @Martin!
Could you please provide us with more details about the troubleshooting you have done so far? I'd follow @WyomingKnott's advice and try the external drive on a PC, if it's pre-formatted in NTFS. Another thing you should do on your Mac is open the Disk Utility and check if the external WD drive is recognized there. Posting some screenshots would really help us guide you through the troubleshooting process. Another thing you should try is swapping the USB cable and the USB port where the HDD is connected to your Mac. If you are able to detect the external HDD in Disk Utility, try
testing the hard drive for defects from there.
Make sure you backup all your data from the drive somewhere else...