[SOLVED] Style sheets not loading

My site http://cphan.org has suddenly developed an odd problem: the Atahualpa theme I've been using for years has stopped working, as if the style sheets are not loading. As a result, all my data seems to be on the site, but without any formatting, headers, columns, font styling, etc.
I've had the site checked for malware and it's been cleaned, and I've checked with my host, who advises it appears that there's an issue with the theme. I've used Atahualpa on half a dozen sites for quite a while with no problems. Any idea what could be causing this or what to do about it? I've deactivated all plugins and nothing changes. I can log in via FTP and I can get into the Wordpress Dashboard. But I can't get it do anything. It's as if the theme has been switched off. When I change to another frame, it all works. Any ideas?
Atahualpa v 3.7.23
Wordpress v 4.1.1