500 error (tried all the usual steps)

So I have the dreaded 500 internal server error on my website. Was doing some innocuous CSS changes on a template file and then whoosh, up pops the error – it’s been like that for an hour at the time of writing.

These are the steps I have taken up to now:
– Deactivating all plugins by renaming the plugins folder.
– Deactivating my current theme by deleting it from the server.
– Re-uploading wp-admin and wp-includes folders from the latest WordPress release.
– Re-uploading the core files from the latest WordPress release.
– Deleting the current .htaccess file and replacing it with a blank.

Further relevant notes:
– The PHP memory limit for WordPress is 256MB. My installation has never threatened to go above about 28MB.
– The work I was doing when the error appeared was modifying a template file that is not currently visible on the live site.
– I’m on WordPress 3.8, which I upgraded to a couple of days ago.

I’m all out of ideas for the time being. Anyone else have a clue on what I might be able to do?