We had a slightly similar incident. We were in process of making some big upgrades in our small datacenter. Just happened to walk by the room with one of my coworkers when we hear a loud beeping from inside the closet. We opened the door and starting trying to find the source of the noise when ALL went quiet. Lost our whole rack, switch, and everyone got to take a 1 hour break while we got it back up. Turned out a breaker had tripped and the UPS ran until it died. Let's just say that monitoring and an additional circuit got higher on the to-do list.