Hôkago senki (2017)

One day after school, Sena Kadowaki falls asleep on the rooftop while waiting for Yudachi Tsutsui to return her handkerchief. When she wakes up, she finds herself trapped in a darkened school. In the school, cut off from the outside world with no signal and no phone service, Sena sees a girl in terrible pain; the young woman has had her hand cut off.