How does Friar Lawrence change plans when he finds out that Romeo did not get his message?

In Act V, Scene 2 of Shakespeare's Romeo and Juliet, the audience learns that Friar Lawrence's message to Romeo was never delivered because Friar John was delayed by a plague threat in Verona. Friar Lawrence's letter details his plans for Juliet to fake her death, be entombed in Capulet's vault and for Romeo to retrieve her at the hour of her awakening. The couple would then proceed back to Mantua where they will remain until tempers die down and the friar is able to announce their marriage. Once he learns that Romeo never receives the letter, he becomes quite apprehensive and instructs Friar John to procure a crow bar so he can force his way into the tomb before Juliet wakes up. He also says he will attempt to get another message to Romeo before bringing Juliet back to his "cell":

Now must I to the monument alone.
Within this three hours will fair Juliet wake.
She will beshrew me much that Romeo
Hath had no notice of these accidents.
But I will write again to Mantua,
And keep her at my cell till Romeo come.
Poor living corse, closed in a dead man’s tomb!

Unfortunately, Romeo's servant witnesses Juliet's funeral and tells Romeo, who vows to commit suicide by her side. Friar Lawrence is too late arriving at the tomb as Romeo has already taken poison. The friar is unable to convince Juliet to leave as she too is bent on suicide after seeing Romeo dead and she ultimately kills herself with Romeo's dagger.