On my computer,
ps -ef | grep Xorg
gives
root 642 632 0 06:09 tty7 00:03:26 /usr/lib/Xorg :0 -seat seat0 -auth /run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
showing that the X server is running. I suspect that when you run the above ps command, that you will get no output, which shows that the X server is not running on your computer. In that case, you need to remove the lock that is preventing it from starting:
rm /tmp/.X0-lock