A poem from the lovely Romeo to Juliet saying that he loves her.
Well, that is a complicated question, because no letter like that is delivered: There was a letter to Romeo talking about the false-death plan, but it was never delivered. Friar Lawrence wrote that letter and Friar John was supposed to deliver it, but was unable to due to a quarantine. Balthasar is the one who tells Romeo that Juliet has died (although she has not... this is the false-death that she will awake from), but he just tells him; he doesn't deliver a letter.
He was quarantined.
Friar John, the bloke that was supposed to give Romeo the letter from Friar Lawrence wasn't able to give it to him. Romeo hears about Juliet's death from his man Balthazar, but not in a letter.
The letter is not delivered to Romeo because it gets wrongly intercepted by Friar Laurence's messenger. This is a conflict related to miscommunication and the consequences of a failed message delivery, which adds to the tragic events in the play.
What letters are there in Romeo and Juliet? Tybalt sends a letter to Romeo's house, which Romeo doesn't get because he doesn't go home. Friar Lawrence sends a letter to Romeo via Friar John which doesn't get delivered. Since the letters are not delivered, they cause nothing. If they are dramatic images of anything, they are images of miscommunication.
Tybalt was annoyed that he wasn't able to fight Romeo at the Capulet party because Capulet stopped him. He therefore wrote Romeo a letter challenging him to a duel. However since he delivered it to the house and Romeo hadn't been home Romeo never got the letter.
Friar Lawrence requests a crowbar to open Juliet's tomb and a letter to write to Romeo explaining the situation with Juliet.
One half hour before the scheduled attack.
it wasn't delivered because Friar John, who was supposed to deliver it was quarantined because of the plague
Romeo is 14 and Juliet is 13.
Friar Laurence's letter is not delivered to Romeo because Friar John was kept in quarantine since there was a plague in his house. Friar John wanted to go to Mantua himself but he couldn't. Once Friar Laurence hears this he says: "Now must I to the monument alone. Within these 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 in my cell till Romeo come." His plan is to go to the tomb, get Juliet then write a second letter. But Friar Laurence's second letter is never sent to Romeo because Romeo's servant Balthazar has already told him Juliet is dead. Romeo arrives at the tomb before Friar Laurence can even write the second letter, which results in Romeo killing himself, which results in Juliet killing herself.
Juliet is the one who is supposed to go to Friar Laurence's cell to meet Romeo for their secret marriage ceremony.