Fatal error: undefined site_url()

A week ago, I was able to access my self-hosted blog. I haven’t changed anything, but now I can’t access it anymore–instead, the following message appears:

Fatal error: Call to undefined function site_url() in /home/90/21/3012190/web/wordpress/wp-includes/general-template.php on line 649

In the .php file, lines 648–650 read

case 'wpurl' :
$output = site_url();
break;

Any ideas as to what might have happened in the past week that would suddenly cause this?

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