18.04 powers off during boot or soon thereafter

I have a Ubuntu 18.04 system that I created to provide remote access to our office March 2020, when the COVID-19 lockdowns started. It has been running great since then … until an update a couple days ago. Now it runs for no more than a few minutes and often powers itself down during the boot sequence without ever reaching the login prompt! That makes it extremely difficult to work on, to say the least.

I used my phone to video the boot sequence (because it disappears too fast to read) and found that when it powers down during a power on sequence, the last message logged to the screen is this:

(  3.144966) fb: switching to nouveaufb from VESA VGA

After that, the screen goes black and the power light on the computer goes out.

Is there any way to recover from this? FYI: I’m no linux/ubuntu expert, so I may need a few more details than are sometimes provided in answers here… (smile) I will try to discover any information requested, but I need to be able to collect it FAST during my very limited time when I can login (and those are few.)