Well, I had Mr. Handy on the 1st floor and it seems he was killed - but there is no body on the floor. I restarted the game, but still - nothing I could revive. Do you have any idea how to get him back?
Sometimes the sprite might be "hidden" behind a pillar or something. Zoom in and see if you can see anything poking out from behind anything, and go room by room.
Alternatively, it's possible that you might have relocated him to another floor accidentally, or sent him out to the wasteland (I don't recall if that is an option however).
And remember, when a Mr Handy dies, it is quite violent, and hard to miss - there is a mini mushroom cloud that appears when he dies, so if you saw one, look around there. If not, try the alternatives I suggested.