Thank you!  I tried recovering by BIOS by holding down CTRL + ESC like it says in that article, but not seeing anything happen.

Looks like Dell Support is closed for the night.  Not sure if they will be open tomorrow since it's a holiday in the US, but I will contact them and open a request when they are.

Any other ideas in the meantime?

If it was working before the update then it could be a failed or corrupt BIOS update. Of course it could be that in fact the MB has a problem in which case a call to Dell tech support is in order. If it is the BIOS then this Dell support page may help.

Tried a CMOS reset, system still won't boot,

You can try a CMOS reset.

I did not see what the update was, unfortunately.  I'm so used to Windows automatically downloading and installing updates that I did not check, just hit 'restart now' when it asked.

Did you notice if Windows downloaded and installed a BIOS update? There is a way in the BIOS to disable Windows from installing a BIOS thru Windows Update. I like Windows but dislike the way Microsoft forces hardware updates on a PC. I can understand software fixes to Windows but they need to leave driver updates to the end user.

You are not the first to come on this message board with a dead system after a Microsoft BIOS update.

how did you do the cmos reset?  did you use the jumper on motherboard?

what video ports are you using to attempt boot?  DP or HDMI?  if DP try HDMI if available.

I tried removing the CMOS battery first to clear the CMOS reset; that didn't work, so I tried moving the jumper as well  Still no change.

I'm using Displayport to boot up; will try to find an HDMI cable and switch to that next.

Update: Switched to HDMI cable instead of DP, no change.  System still not booting; 2 yellow flashes on the power button, and then 1 flash.

That sounds very much like a BIOS update problem. Mine did something similar when it auto-updated from 2.1.3 to 2.2.1 a couple of months ago. 
At least in my case I did get display, but then the Windows spinning dots would appear and nothing would happen. And I also got the 2,1 POST light.
Tried every solution under the sun to fix the BIOS, but in the end, only a motherboard replacement and changing the BIOS to not Auto-Update fixed it. Even now, I untick the box in the Dell support program for BIOS update, because even though we're at 2.2.4, I have no faith the problem is solved.

At this point I would contact dell cares for assistance:

Please do not post your private Service Tag in public. Click the blue DELL-Cares username. Their profile page will appear. On the right click, "Send this user a private message". They will need the private PC Service Tag to check your purchased warranty status, components, etc.