Q: IMAC (RETINA 5K, 27-INCH, LATE 2015) startup chime and blank screen

HI,

I initiated the latest os update this evening, I returned after and hour to find the screen "on" but black.

Both mouse and keyboard seemed disconnected even though both have charge

I forced shutdown by holding rear power button, then disconnected the power cord for a minute before plugging it back in and powering back on

The startup chime sounds followed by nothing but a blank screen

Any suggestion for a "quick fix"?