This time a device called a PHP Opcode Cache was at fault. The particular piece of software in question is APC.
Some explanation is in order. WordPress is written in a language called PHP. Each time a page is loaded, the PHP program reads the programming language file off the disk and transforms it into a series of intermediate formats. The last of these is the opcode, which is then executed by PHP.
An opcode cache keeps this final product in memory, so that PHP needn’t reload everything from scratch for every request. It improves performance by speeding up PHP and also reducing the amount of disk traffic.
Apparently, some time this morning, APC decided to chuck a wobbly and stop working. I’ve uninstalled it and replaced with a different opcode cache.