Computer data is corrupted when it becomes unreadable. There are several common causes.
The writing of adata to a file is interrupted. This can happed when a USB or disk is removed while it is being written to or when a computer undergoes a 'hard shutdown' - when the power cord is pulled out.
Hardware failure 0 a failing disk or overheating CPU.
Programming errors
Interrupted or incorrectly implemented System upgrades
An encryption key is corrupted. Opening a file may corrupt it
A corrupted disk can result in files going missing, even if the data is still on the disk. Later the file may be overwritten and lost forever. Technologies such as File System Journaling have helped to reduce the potential for directory corruption due to power outages or hard restarts. Likewise, while hard drives have become exceedingly reliable, they are still known to fail catastrophically with little or no warning.
Corruption cannot always be prevented, but the effects can be reduced.
Many systems implement daily, weekly and monthly backups of all data.
Disk drives can be RAIDED, so that multiple copies of each data change are made
Data can be written to a completely different media, such as DVD.
IN the end no method of data storage is secure from corruption. Paper can burn or rot, DVD's can break or made unreadable by UV rays, and magnetic tape by electromagnetic fields. It will be safest to store the same data in multiple formats and distribute it widely, but then of course, it is not your data anymore
Once data has been corrupted, it can often be at least partially recovered. For example, a mysql database is made up of tables which can be reconstructed even after corruption, often in a few minutes, by executing a terminal command