So you have updated the kernel on you centos but you server is still running on the old kernel. Whenan kernel is updated yum updates the file /etc/grub.conf and that is an synlink to /boot/grub/grub.conf but if the link is broken then you will have two grub.conf. One /etc/grub.conf and one /boot/grub/grub.conf and then when you update en kernel the server will still not run on the new kernel.
Fixing this
cd /etc mv /boot/grub/grub.conf /boot/grub/grub.conf_old mv /etc/grub.conf /boot/grub/ ln -s /boot/grub/grub.conf .
Now the files are correct restart you server and it will start up on the new kernel
reboot