Hi Mike, I hit the exact same error — VFS: Unable to mount root fs on unknown-block(0,0) after a kernel update.
Root cause in my case: missing initramfs. Ubuntu’s NVMe driver (CONFIG_BLK_DEV_NVME=m) only exists as a module inside initramfs — without it the kernel can’t see any storage at all.
Immediate fix: Boot from an older kernel in GRUB menu, then run:
sudo update-initramfs -u -k <your-affected-kernel-version>
sudo update-grub
sudo reboot
I investigated this deeply and reported it as an upstream Ubuntu bug. The root cause is a silent exit 0 in 55-initrd.install when initramfs is missing — GRUB creates an unbootable boot entry with zero warning to the user.
Bug report (Launchpad): https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/2141741
Full investigation notes: https://github.com/jillravaliya/kernel-panic-study
Hope this helps!