Crashes of a computer, server or network are possible at any time, even though they may not be frequent. I have seen a number of files which have apparently been saved under normal circumstances on a good floppy, but which can no longer be opened by the program - the only answer is to retrieve a previous version. Large or complex files with many graphics are most at risk. Raw text can often be extracted from damaged files. BBedit can be set via a pop-up menu to open any file.
Graphics are normally unrecoverable.
Disk First Aid is a utility you should run from time to time to examine your disks and remedy any inconsistencies before they could cause loss of data.