Why does it occur ?
Logical Crash
Most data loss occur due to Virus, Worm or Trojan that writes garbage in the MBR (master boot record) of the hard disk, making it impossible for DOS / WINDOWS to recognise the logical partitions. More dangerous Virus, even destroy the Boot Record and FAT (file allocation tables) as well. The worst are those which formats the Hard Drive (low level) or fill the files with garbage and set the file length to Zero. Same is the case with Floppy.
Physical Crash
Physical crash is a mechanical failure of the Hard Drive. The hard disc rotates at 5400 - 10000 RPM. Thus with time scratch develops on the Media or the Head is broken due to excessive friction. In this case The Computer BIOS doesn't recognise the hard drive altogether. Same is the case with floppy. Although it rotates slowly the media of the Floppy is very fragile and develops scratch with time.