Upvote:3
A Schengen visa can be extended if departure is prevented by "force majeure" such as unexpectedly closed airports (see the link in the question that was tagged as duplicate). The reason why I write this answer is one important addition: The application for an extension should be done before the visa expires. If you are already at Frankfurt airport, you might ask the Bundespolizei if there is a visa office open on a Saturday evening.