Mac Flashing folder questions mark

Recently, while turning on my mac I was surprised to see a flashing folder with question mark. And I was not able to log in to my system. So, I started investigating the cause of this problem. And there may be 3 causes:

1.Damaged HDD connector.
2.Corrupted HDD.
3.OS has been corrupted.

In most of the cases it is the HDD connector which got damaged. 

So how can you check that one ? Well, If you have an external hdd connector, then you can open your mac HDD and connect the same externally.

  Now, if your system boots up, then its a problem with your HDD connector. If you still have the same problem, try                  connecting the mac HDD to another machine (preferably linux) externally and check whether your file is showing or not if you   are able to see any file then your os has been corrupted. Otherwise it is a problem with HDD.