I've had the same problem happen a few times, on several different EE sites, but I can't determine the cause so I'm turning to y'all.
Symptom: The site's main index page will fail to load and will display a blank white page with no output. When diagnosed, the problem turns out to be that the default template group "forgot" that it was the default template group. More specifically, the "is_site_default" field in the exp_templates table, was set to 'n' for all template groups. (In order to function, it has to be set to 'y' for one template group for each site you are running.)
Has anyone had this happen to them, and if so, do you know what causes it, so I can avoid it recurring?