Common causes for data loss

Software errors
Data carrier was formatted and/or partitioned
Data or directories were deleted
Data are only shown to have a size of 0 bytes
System was newly installed or data and/or directories were overwritten
Directories or data suddenly vanished or cannot be found anymore
Data or directories were copied in a wrong direction (eg backup to data)
Errors in the data system, the operating system wants to conduct a system check.
Password was forgotten (eg harddisk HDD password, bios or data password)
Computer viruses
A data medium (eg harddisk, flash) is suddenly empty, the operating system wants to format it.

File errors
A fault in the data base (eg MS SQL Server, Oracle, MySQL) has occured
Mail data base (eg Outlook, Outlook Express, Exchange Server) is faulty
A document is not represented correctly anymore (eg Excel, Word, Powerpoint, Autocad, etc.)
An archive (eg. ZIP, Tar, NT Backup, Veritas Backup) is faulty

Raid
Raid network is indicated as being "critical" or "offline"
Disks vanished from Raid network
Rebuild of Raid network terminated
Raid network was reinitialised or rebuilt faultily

Hardware errors
Error message "no data carrier found" or "no system found"
Server or PC does not star up
Hard disk makes clicking or scratching noises
Hard disk does not spin
Hard disk "tingles"
Hard disk overheats
Damage from fire or water (sweet water, salt water, rain water, etc)
Voltage surge occured, electronic parts smell burned or chips turned black
Data carrier was already opened by non-specialist
Tape damaged
Optical medium (eg CD, DVD, BlueRay-Disc) is faulty
A flash medium (eg USB stick, CompactFlash, SmartMedia) is faulty