500 Server Error – index.php no longer loads

My index.php (i.e., the blog homepage) no longer loads; instead I receive the 500 server error message.

I’ve poked around the forums, but I’ve had no luck with either the memory=20MB trick or editing .htaccess.

I can access the dashboard, and I can retrieve the actual rss feed (e.g., ?feed=rss2), but the index.php file returns the 500 Server Error.

The only plugin I’m using is Akismet (which I tried de-activating just in case – doesn’t fix anything). WP is version 2.7.1.

The problem seemed to have started today, which doesn’t really make any sense to me as I haven’t made any changes in about 2 weeks.