kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

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A linux kernel will not boot with the following error message:

kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

There can be many different reasons why a kernel cannot mount the root partition. Help us document them here, in order of likelihood, with solutions for each.

https://wiki.gentoo.org/wiki/Knowledge_Base:Unable_to_mount_root_fs