If you upgraded from CentOS 6.0 to 6.1/6.2 via YUM, etc:

edit /boot/grub/grub.conf and /etc/fstab by changing

/dev/xvda1 to /dev/xvde1

and

 /dev/xvda2 to /dev/xvde2

To ensure the server can boot.

(You can perform this change via the Console or recovery Console if your server is not booting or getting online.)