Thanks for the reply guys! Fortunately, my hard drive was restored. 
I’ve tried all your suggestions, as well as suggestions from various online fora, such as this. I’ve even contacted Apple Support to help me solve the problem. But it turned out, I just explored the disk utility application. I repeatedly opened the app due to its numerous crashes, which made me opened related applications such as, Time Machine and Startup Disk (by the way, crashes on Disk Utility occurred right after I updated to OS X El Capitan) After a while, viola! My hard drive appeared on my desktop, as well as on Finder. I was very happy that all the files are intact! 
This may not pinpoint a specific solution to a problematic HD, but I just wanna suggest that you extend your patience with your HD, and let it rest for a while. To possibly avoid future issues such as this, I ran the First Aid button on Disk Utility for repairs.
Macbook Pro Retina Display
Late 2013 Model
WD My Passport Ultra, USB 3
