I Slept in the Apocalyptic Ward for 900 Years

A bell tolled, causing the world to fall into an unknown slumber. When I opened my eyes again, a man with a military ceiling appeared in front of me. He told me that this was the first day of the 900th year of the new era. I came to this new world inexplicably. I entered the best high school, joined the alliance organization, and went to explore all the new things in this new world. Only later did I realize that behind the ringing of the bell, I was closely related...