Accidentally did "chmod -777 / "

Note that chmod -777 is like chmod 000. So the OP has probably just changed the permissions of the / folder, not recursively, and he or she can recover the system simply with a

chmod 755 /

from a recovery boot. For example, boot from the installation media, choose try Ubuntu, open a terminal and go superuser (do the following checking thrice and only after having understood what you are doing)

sudo -i
mount /dev/yourrootpartitonhere /mnt
cd /mnt
chmod 755 .
cd ..
umount /mnt

And reboot normally, all should be OK.

If you did use -R, do a backup of your data (see other answers here), and reinstall.

Thanks for your answers.

I freshly installed Ubuntu 14.04, however I was able to backup my data using live CD.

I booted from a live CD and I did chmod 777 / 'pathOfTheDirectory'

Do not mess with things in which you don't have much knowledge - lesson learned.