But then computers so often don't ...
OK, so earlier today I upgraded my WordPress theme, Tarski, in preparation for an update to WordPress itself. After 3 hours, much cursing and many broken plugins, I mostly got things working and looking the way they were before. Then, about 6 hours later, I noticed that the Similar Posts plugin also wasn't working, so (hoping for a quick fix) I upgraded that to the latest version. This turned out to be a bad idea, because then all pages started showing up completely blank -- including admin ones. Presumably Similar Posts did something bad to the php then: that happens sometimes. So I went in with ftp and deleted Similar Posts. No change. I deleted the old Similar Posts too. Still no change! Now I'm worried that the mysql database has been hosed somehow (which would serve me right, as I decided I couldn't be bothered backing it up earlier in the day). I reverted to the previous version of Tarski, the one I'd been using before I touched anything today. No change. Finally I reverted to a really old version of Tarski, some six months old, and that finally undid the damage. (The WordPress Default theme works as well.)
So the database actually is ok (and is now backed it up, of course ...) But this all makes no sense. It was clearly Similar Posts which caused the whole problem, so why did I have revert Tarski to an ancient version to fix it? Bleh. I was quite happy with Tarski, but the way it's being developed means that I have to rewrite more and more of it in order to make it do what I want. So maybe I'll have to look around for another nice, clean theme.
It might just be some sort of caching thing, or I might have another look tomorrow and figure out what the problem really was, but for the moment things Airminded will not quite be itself until I can do something about it. Apologies for any inconvenience!
Update: fixed now. The problem was some extra code relating to Similar Posts which was residing in Tarski's constants.php file, which explains why I had to go back to an old version of Tarski (when I wasn't using Similar Posts), but not why going back to the previous version of Similar Posts didn't work, when it had been a few minutes earlier ... But as a bonus, in the process of getting Similar Posts to work again, I now understand Tarski's new way of doing things better, so it has a reprieve :)