In the Note Grid you can choose to display the notes in a Chromatic view, where all notes are laid out chromatically (all note values are displayed), or in Scale view, where only the notes in the currently selected Scale are displayed.
If you should change to another Scale, some notes might "disappear" visually in Scale view and are then indicated with thin red lines:
To make all notes visible again, simply change to Chromatic view on the Settings menu:
Tap the Settings button:
Tap "Scale" in the Note Grid section: