Here's a fun error:
"not syncing vfs unable to mount root fs on unknown block"
I'm running CentOS7 on Linode and happened to run into this error on one of my systems. Quite unusual, so I did the quick and dirty method and restored a backup of my VM. It's pretty much stagnant, so it's not a big deal to lose a few log files. So I did, and guess what? No luck. Time to actually troubleshoot.
Luckily, Linode comes with Configuration options for disks, including Kernel selection. I was trying to modify the grub file directly using fennix, their delivered recovery tool. Luckily I didn't need all that. Changing my Kernel from GRUB2 to Latest 64 bit allowed me to finally boot.
It's a quick fix. Maybe not a permanent one, but for now, we'll stick with it.