If you recently updated your Centos VM you may started getting the error below:
Kernel panic -- not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
unknown-block may change depending on your configuration, but symptoms stay same: Kernel panic after the update, and this error message.
Solution is:
yum remove kernel yum update
That fixed my problem. Hopefully it fixes yours.
note to self 