A component's template threw. An error inside a component or a linked record does not stop the rest of the page from rendering, so a page whose whole body is one component can render blank. Look under Template Errors in the admin, or open the page on the development environment where the trace shows. Other blanks: the site's own 500 page failed while rendering (keep fatal-error.html self-contained), or the content sits inside a {literal} that was never closed.