Why didn't Romeo get the message Friar Laurence sent him? Because Friar Laurence sent the letter with his friend, Friar John, who was delayed due to an unfortunate mix-up. (Someone accidentally thought he had the plague).
Friar John comes back without having delivered the letter. Friar Laurence has a bad feeling about this.
Friar Laurence goes off to the tomb thinking he'll have to wake Juliet alone.