trying IRC right now, but no dice so far. GoDaddy told me they can't help me - that it's a silverstripe issue.
I'm now at the point where I've read main.php and learned from it that I can at least access my files by adding /sapphire/main.php?url=... to the address, but still can't get the site to work properly.
Thanks to DesignCity, Tweak, stuckinrealtime and Psykick, problem's solved! It probably all began when I mistakenly changed the homepage's url, then edited .htaccess, ...then it kinda snowballed from there.