"/etc/X11/X not executable" error when startx

you have to try this

 sudo dpkg-reconfigure -phigh xserver-xorg

I am writing an answer for my question, but actually I haven't figured out exactly what happened that gave rise to those errors.

I am assuming that the whole xorg itself broke and I got my desktop back by using a live disk and chrooting (yeah - had to do that as its hard to connect to internet from terminal and its lot more easier to connect to internet from live disk and copy resolv.conf from here to there).

So, after creating a live disk and in Try Ubuntu mode, I chrooted into old partitions (I followed the guide here) and did

apt-get update && apt-get upgrade
apt-get install --reinstall xserver-xorg
apt-get install --reinstall gdm gnome-desktop-environment xorg

Then I changed my default display manager to gdm at /etc/X11/default-display-manager by changing this line (I think this step is not needed if after a restart your desktop is working fine, but I basically did a couple more things than said above and broke lightdm too, so I had to change to gdm!)

/usr/sbin/lightdm

to

/usr/sbin/gdm

and restarted. Well, unity is working and I got my desktop back working.