Data loss is an event that happens when a file, program, or a piece of information is lost. When data is updated, it may change, and this may not be data loss. Data loss also does not occur when the data is unavailable for some time only, for example because the server it is stored on cannot be reached. When this server can be reached again, the data will again be there. There are some common causes to data loss. Data loss can sometimes be prevented by creating a backup copy. A backup copy may contain an older version of a file, which can make data loss less severe. If certain information used by the system is lost, the system may stop working. Getting the system working again and restoring the data from a backup copy may take a long time.