Answer:
The friar sent to give the message to Romeo was quarantined, Romeo thought Juliet was dead, so drank the poison he got from the apothecary. Juliet's father moved her and Paris' wedding date up, so Juliet panicked and devised a plan to drink the sleeping potion that made Romeo poison himself. Romeo killed Tybalt, Juliet's cousin, because Tybalt killed Romeo's friend, and for that Romeo gets banished, and Lady Capulet thinks of a plan to avenge Tybalt. There's probably many more but I cant remember off the top of my head.