I'm not sure there's a way to delete it or add back an individual light.
Is it possible you grouped it with another light?
or that it shifted to another room, maybe the default room on it's own?
or maybe in the hue app it's in a room that doesn't exist in the home app.
if you ask siri to "turn on all lights" does that light come on?
does it respond to other commands to all lights?
if so that means it's in homekit somewhere
if you go into home settings, (click the house in the upper right) and go to the bridge, and open the accessories option, is the light listed? they're grouped by room, and you can control it there.
You could also try rebooting your phone.