CentOS 6.9 GUI problem

[KALYAN]

I have updated my CentOS desktop and tried to restart(shutdown -r now). But I lost the GUI. I tried ctrl+alt+Fn and also
a) #yum -y groups install "GNOME Desktop"
#startx
b)#yum install lightdm
c)#yum group install "X Windows system"

but still facing the same problem (only command line interface is working). Please suggest me a solution

[TrevorH]

Had you previously installed proprietary video drivers from nvidia/ATI/Virtualbox/VMware et al? If so then they will need reinstalling after every kernel update unless you use the packaged version of the nvidia ones from ELRepo.