While upgrading a headless Mac Mini 2018 to Big Sur the upgrade stopped. I connected a monitor and saw a picture that looked like the side of a laptop with a green dot in a small slider on the left. Under that was an arrow pointing left. That alternated with a picture of a mouse. Under the mouse was a circle with another arrow.
I connected a mouse and clicked the arrow under the mouse picture. It was replaced by the picture of the laptop side. I could not click anywhere on that. After waiting a few minutes with no change, I pushed the power button to turn off the Mac.
When I pushed the power button to turn it back on nothing happened. The LED light on the front was blinking. It started flashing as soon as the power was plugged in, whether I pushed the power button or not, and did not stop until I unplugged it. The LED seemed to blink about nine times, pause, then repeat. Eventually I found a page on Apple’s site that showed exactly what I saw. The description said the Mac was in Firmware Recovery Mode.
That section had a link to instructions on how to revive the firmware, and that page had steps for a Mac Mini. I connected it to another Mac with a USB-C to USB-C sync & charge cable and followed the instructions. It took a few times before the Mac Mini showed up in Apple Configurator 2, but then it ran and after a bit the Mac rebooted with a welcome chime! It resumed the upgrade to Big Sur, which completed successfully.