You can get an idea of what is going on in a drive and why we use a class 100 clean room. On the following picture, you can see the sizes of a human hair, a finger print, regular dust and how close the head float from the surface of the platters.
The head floats on an air barrier generated by the rotation speed of the platters at about 0.015mm. A humain hair diameter is between 0.1 to 0.3mm. You can get an idea for finger print and dust particules. Hence the glove for the hand who works on the drive and the class 100 clean room.
To put things on a bigger scale, the head could be seen as a Airbus 380 travelling at 2500Km/h at 1.5mm from the ground. The dust particules or the finger print would be seen as a massive mountain. In this way, you get a pretty good idea of what a head crash looks like.
This said, there is an important difference with the plane analogy. The coating on the platters get damaged when the head crashes. This is where your data are stored magnetically. Luckily not all head crash will damage the platters.
You can now imagine that if your drive fall on the floor while it is working what kind of damage can happen. The size of a scratch on a magnetic platters could be from the size of a hair to much more. Which explains why most recovery on drives with scratched platters are impossible. The head would literally explodes while flying this scrated area, making even more damaged.
After a fall, one of the symptom, could be a hard drive clicking. This is a typical if the head or the preamplificator has been damaged. It is also typical, if the system area had been scratched. The system area is located on the platters.
Check our database hard drive clicking sound below: