Mercifully, a Mac genius on the Apple support site was able to guide me through digging into the actual code of my iMac's operating system, allowing me to bypass the loop that was locking me out. I've had to reformat the whole computer, but my old files were preserved: now its more a matter of picking everything up and putting it back in place, or like moving into a new apartment. My most-used internet applications – Netscape, AOL – seem to be resisting booting up, but that may just be a matter of re-installing those as well. Compared to not being able to get the computer itself to open up, it's nothing: it should be fairly easy work to straighten out.
In the meantime, I've not yet gone to bed from Friday yet: I don't know what it is about working on computers that so easily lends itself to neither eating or sleeping, but I fell under its spell again. Other than putting the final polish on some of my travel journal entries, which I've started uploading, I've been digging around in this computer since Friday evening. Time to take a break!