It looks like I’ve learned my first lesson with WordPress: when upgrading, proceed with caution! A recent upgrade for NextGEN Gallery plug-in, which powers the Portfolio page and part of the home page, caused it to fail: I had no screenshots showing up at all.
Luckily, the fix turned out to be an easy one (I needed to re-upload my custom template files, which were deleted with the upgrade). But from now on I’ll proceed with caution before clicking any automatic “update” link.