A crash is one of several hardware failures that may strike a disk, that will damage your files or make them inaccessible. This causes scratches in the magnetic coating of the platter. The damage caused by a crash can vary in extent, from damaging a few sectors to removing all the coating of one or more sides of the platters within the hard disk. The files that where stored in the areas where the coating was removed cannot be recovered by anyone or anything. Other typical hardware failures are damage to the disks mechanics or electronics.
Disk crash is the most frequent cause for data loss.
If your drive is physically damaged you may increase the damage by reading it only. To illustrate this we will give you an example. When a drive is dropped, the heads are going to crash against the surfaces immediately below them. If done instantly, all the data can most likely be copied off the disk without any problem. However, what normally happens is that the disk is picked up and used for some time - until the day the problem has grown to one that you become aware of.