My guess is she experienced a big emotional breakdown at work that she was taken to a mental health facility where she received medication.  Psychiatric problems... Maybe she yell or fought her boss out of exhaustion and frustration  and they put her on a forced leave of absence so it is linked to the scene where we see her that she is back in Korea (when Choi Seung is kissing his ex girlfriend inside of the car). 

Watching Ep 7, I am guessing it could be miscarriage due to her excessive work style. The way SR and her ex was talking...Her break does seems be related to them now.  Ahhhh!!! my brain isn't working and I am literally so curious to know the actual reason for her break!!!!! 

why are you guys so good at guessing? I hate the sick card that the writers pull. Why don't they just use the miscarriage plot? It sounds a bit better :" at least it won't resurface...