Whew! That was a frightening day.
I don't know how many of you were lucky enough to try to view the site this morning and this afternoon, but...it wasn't there!
My hosting provider did a php upgrade last night, going from version 4 to version 5. Little did I know, but my Drupal backend was not compatible with version 5 of php (I'm using a pretty old version of Drupal for this site).
Lots and lots of googling later, I finally managed to upgrade the drupal install using a backup of the mysql tables I made earlier.
What I've learned from all this, though, is that I have my base install of Drupal done in precisely the wrong way. I really need to change how I have it set up on the server. Maybe I'll start on that tomorrow...but until I get that migration underway, at least the site is back up and seems to be in working order.