equestria-28 days later

equestria is a barren wasteland, the infected roam the countryside, our hero's must travel to canterlot to meet princess celestia who has the answer to infection

when a human falls into a dimensional portal and winds up in Equestria, he finds it barren of life, his brony instincts kicks in and he heads to golden oaks library, what happened? why is he here? and what are those noises he can hear at night?