From the beginning of the play up until Act III scene v, the Nurse is Juliet's only friend.  She is actually her care giver--since Juliet's birth she has served in the capacity of what we now call a nanny.  She is more of a mother than Lady Capulet has ever been.  The Nurse helps her in every aspect to get together with Romeo--including both their secret marriage and their "wedding night" together.

Because Juliet has no one else to turn to other than the Nurse, she finds herself completely alone after Act III.  In Act III scene v, the Nurse is finally put in her place and reminded (by Lord Capulet) that she is an employee of HIS.  Her job is to be the adult and follow his orders.  Up until this point in the play, the Nurse has gone behind his back and set Juliet up with his enemy's son.  Because of this scene, the Nurse encourages Juliet to forget Romeo and take Paris' hand in marriage. The lines

"I think it best you married with the county.
O, he's a lovely gentleman!
Romeo's a dishclout to him"

And then Juliet's one word response of "AMEN" shows how she cannot trust the Nurse any more.  The fact that the Nurse was all Juliet had then makes her isolation so much more dire.  She has no one, and the Friar 's plan for her to pretend her death only isolates her more.  After she fakes her death, she is literally alone until the closing of the play.  Had her Nurse realized her error or perhaps still helped Juliet in some way get out of her trouble, Juliet wouldn't have been alone.