To begin a short explanation on how the data are stored on the hard disk :

When you save : when you save the data is stored on the magnetic coating of the platter of the hard disk.

This magnetic coating is the material that keep your precious data, then if this magnetic coating is damaged you will suffer from a loss of data. Each part of missing coating contains data will correspond to data loss, thus more it misses of coating more the recovery will be difficult.

Which parts can fail in a Hard Disc?

-Printed Circuit Board (PCB)

 The Printed Circuit Board of the hard disc is the electronics of the hard drive that manages and operates the hard drive.

-Platters

 Platter damage is the most serious type of damage to cause hard drive failure.

Any surface contact to the platters can cause irrevocable damage to the magnetic coating contains data. This includes dust, finger  prints, water.

-Firmware

  Firmware is a program instructions located on the chips of the Printed Circuit Board (PCB).

-Heads (Head Crash)

  If accidentally a head touches a platter then a Head crash occurs.

There is many recovery software on the market, but as you could noted some damage is irremediable, it is thus more reasonable to go for a backup solution, because you know what you can lose, but you do not know what you will recover, this is why I still advise to make backup regularly.

Think well before a disaster occurs.