Notebooks 6 was discontinued in December 2013, when we introduced a paid update, which is Notebooks 8 by now. Notebooks 6 used to run fine up to iOS 10, but Apple decided to cease support for older apps, so Notebooks 6 will no longer launch on iOS 11. Over the past couple of months, iOS 10 should have notified users about that.
We don’t have an option to update Notebooks 6 and make it compatible with iOS 11, so the recommended solution is to move to the latest version of Notebooks.
We have created a migration guide describing how to upgrade and move your documents from Notebooks 6 to Notebooks 8.