It is inevitable. Hopefully it won't be the Ph.D. thesis or hit novel you've been working on for years, though I know people who have lost such things. Apparently the U.S. government lost all census data for a particular year in the 1970's because the computer disks got corrupted and there was no paper backup. Take my advice. Save your work often, and keep back-ups of important files in several locations.
Retrospect is a great program that automates data backup.
However, there are several programs on the market for recovering files that have been damaged or accidentally deleted. They don't always work, but it's often worth a try. A popular cross-platform commercial program for disk repair is Norton Utilities . There are entire books written on the subject, which is fascinating in a twisted sort of way. If you want these programs for your Macintosh, it is legal to copy them and pass them along in the interest of keeping the computer world virus free.