Ubuntu. Driver broke, all of a sudden, can't communicate with device

Ubuntu 18.04, RTX 3080, Nuvo-8000 series EPU

One day, without any changes made to nvidia/cuda installs, the computer stopped being able to launch a graphical desktop, just giving a black screen. Booting into recovery mode or switching to a TTY confirmed that the nvidia driver was not able to communicate well with the GPU. I’ve seen this problem before triggered by an unattended upgrade to the kernel. I tried booting an older kernel and also purging and reinstalling various different nvidia drivers, 470, 525, 530, 530-open, others I forget… and trying combinations with different recent and not so recent kernels but nothing fixed the problem. The output from nvidia-smi was always “No devices were found.”
attached in the nvidia-bug-report
data (2.8 MB)

there seems to be a lot of this (?)

9.163520] NVRM: GPU 0000:06:00.0: RmInitAdapter failed! (0x26:0x56:1253)