It's an important exercise to think this ahead. I did it last year, telling each client, "in case I get run over by a semi..." your details are in "readme.doc" located on your website. Therein, I placed potent details on domain registrar, DNS assignments, hosting company, folder names on my computer, backup copy locations, and such.
Now that I'm facing a recent HPV diagnosis, I find it important to teach mi esposa to maintain her own website (same "semi").
Such ahead-of-time documentation is a good motivator toward a more simplified, efficient website setup, too.