I have no idea if this goes in the PHP subforum, since I have no clue what the problem even is, but here we go...
I run a standard Joomla website installation on an on-site server, using PHP 5.2 and Apache 2.2. It's a fairly large website (several thousand content-rich pages), and as many of you may know, Joomla's default search engine isn't necessarily the best for finding pages in this sort of site. Given that, I tweaked the search module to add another option, order by "relevance"--Joomla's default is order by date modified, which in our field really has no relation to what a user was searching for. The tweak worked perfectly, and has existed for well over a year with no problems, including going through a site redesign and several major structural changes without issues.
However, today when I got into work, I used it to look up a page--and realized it wasn't working; not just for the particular search I was trying, but for any searches whatsoever. Using the search feature brings up a blank white page, and using Firebug to look at the HTTP headers indicates that the server is throwing a 500 error.
All well and good, this is why I keep PHP error logs. So, I look up the logs... Nothing. No errors listed. Odd... perhaps it's a Joomla error? No; the Joomla error page hasn't tracked any errors. Apache error, perhaps? Nothing there, either--the Apache error logs are clean as a whistle. The rest of the Joomla website is working in its entirety; to my knowledge (as the only programmer that maintains this site) nothing changed over the weekend.
So, my question to the WebmasterWorld audience: what on earth might be going on here? Without any error logs noticing anything, or any error message displayed to the screen, or... well, any information at all, I'm at a bit of a loss to figure out what on earth might be happening. Anyone have any recommendations to check for what might be happening and why?