Friar Laurence’s letter never reached Romeo because its messenger Friar John was stopped by a plague.
When Romeo was banished for killing Tybalt, Friar Laurence tried to be in communication with him so he would not be cut off completely from his new bride, Juliet. One of the most serious problems that occurred right off the bat was Juliet’s forced marriage to Paris. Juliet went to Friar Laurence for help, and his solution was to fake her death. This was only a good idea, though, if Romeo was in on the plan.
Friar Laurence wrote a letter to Romeo explaining what was going on. Imagine his surprise when he found out that it never reached its destination. Friar John arrived, and told Friar Laurence the bad news.
Going to find a bare-foot brother out
One of our order, to associate me,
Here in this city visiting the sick,
And finding him, the searchers of the town,
Suspecting that we both were in a house
Where the infectious pestilence did reign,
Seal'd up the doors, and would not let us forth;
So that my speed to Mantua there was stay'd. (Act 5, Scene 2)
Friar Laurence asked the obvious question: Who took the letter to Romeo then? The answer, unfortunately, is no one. Friar Laurence was upset, and told him that the letter was important. He feared that Romeo would do something rash, because he knew Romeo.
Romeo did do something rash. He got himself a vial of poison, went to Juliet’s tomb, and poisoned himself. Juliet awoke to find the husband she had gone to so much trouble to stay with dead by her side. Devastated, she committed suicide with his dagger. Naturally, Friar Laurence blamed himself. It is another example of circumstances really being against these two. They pushed fate and lost. You could say they had really bad luck.