At the end of the preparation rites, just before the preface dialogue, the musician(s) wake up (or sleep walk) and start up the Sanctus. Before you know it, the music is done and everybody is kneeling. There are some choices:
A. Go back to the preface dialogue and skip the Sanctus
B. Go back to the preface dialogue, and sing the Sanctus again when indicated.
C. Forego the detour and continue with the Eucharistic Prayer as if it was you yourself who fell asleep.