failed to mount root filesystem on unknown-block(0,0) means the kernel started, loaded builtin drivers and drivers from the initrd, looked for the system partition to continue starting up and couldn’t find it.

Maybe you removed a disk and /dev/sdb became /dev/sda or maybe you forgot to add nvme SSD support and the kernel can’t read /dev/nvme0n1p1.

Or your disk let out the magic smoke and isn’t detected any more.