Luckily, it happened with my Netscape 8.1, just a few days after I had installed Netscape 9.0, which had imported a copy of 8.1's bookmark file, so all is not lost. Since I don't think I added any bookmarks in the past two or three days, I don't think I lost anything important.
Still, it's rather annoying, since there was no reason for it to have occurred. One moment it was fine, the next Netscape looked as though I had just installed it for the first time, having reverted to its default theme with Netscape.com replacing my usual homepage.
I ended up installing the latest iteration of that particular build, version 8.1.3, since Netscape 9 doesn't seem to have 8.x's dual rendering engines, which can be useful sometimes for certain sites that seem to hate Mozilla/Firefox. (*cough* LiveJournal *cough*)
I was lucky this time that I didn't lose all my bookmarks, so I shall have to be more anal-retentive than in the past about backing up bookmarks regularly and often, as well as other files. Still, I am rather perturbed that it happened for no known reason, right out of the blue. Makes me wonder when the next time something like that will happen next . . .