How is it caused?

The head crash is it caused when the read-write head of a hard drive comes in contact with the platter. When this happens, a read-write head destroys the magnetic coating on one of the platters, then small particles of dust are expanded causing other heads to crash (in multiple platter drives) and the same process happens again and again until debris causes such extensive damage, that there is no magnetic coating left on the platters.

How can we avoid it?

There is a very easy way to avoid this failure: take care of your devices. Every time that you put the laptop down heavily on a table, or you drop it in the sofa because we think that the surface is soft and nothing it is going to happen, actually we are not being protective enough with our media as small shockwaves can penetrate the hard drive and contribute to a head crash.

The other way to protect your data is to ensure that your data is backed up.

What if I’ve already suffered a head crash?

If you think that you are in this situation, firstly don’t panic. Then head caution if you decide to search for advice on how to fix your hard drive if this has suffered a head crash. This is a delicate process to recover data from and as such we do not recommend that you attempt th ese practices if you value your data. The reason is simpl e: you probably don’t have the right tools and the experience to affect the best possible recovery (if you do you are probably not reading this blog post!). When you have a problem with your car, you go to professionals, right? The same happens with your hard drive, if you start “fixing” your hard drive you can make the damage worse . Our best advice is call to a reputable data recovery company; they will give you better advice and an evaluation about your hard drive and how much data can be recovered.

Reference: http://blog.krollontrack.co.uk/top-tips/hard-drive-head-crash/