Friar Lawrence gave her a mixture to pretend her death, because she was going to get married to Paris.
We see that in the story, Romeo and Juliet have already gotten married when she is told by her mother and farther that she will be getting married in 3 days, or in the story I believe it's Thursday. She cannot get married to Paris because this is a secret she is married to Romeo, therefore Friar gives Juliet a mixture that stops her heart from beating and looks like she's dead, but really she's just in "hibernation" until Romeo comes back.
I hope this helps! :)
