AF-Assist illuminator problems with D810

Hi Nikonites,
I turned off the AF-Assist Illuminator on the custom setting menu (for SCUBA photography application). When I turned it ON again, it didn't work. Tried it few times, turned off the camera, all in vain :blue:
Anybody knows how to bring it back to life?
Thanks

Camera mode has to be AF-S, and if NOT Auto Area, the focus sensor has to be the center one.
D810 manual page 313.

Thank you Wayne! This was very helpful (I was on S mode). Problem resolved!!!