So I am using ubuntu 17 with GNOME and I can't login in anymore on my laptop. I have looked up this issue and I have tried several of the methods suggested:
None of these methods works, whenever I try to login, the screen goes black for a few seconds and return to the orginal login view.
This is a notorious bug. This is what helped me solve the problem. I was running Crash Plan (a backup service) on my computer which was consuming the directory watches. Take a look at Failed to add /run/systemd/ask-password to directory watch: No space left on device?
Try install gdm
crtl + alt +f2(login)
sudo apt install gdm
sudo dpkg-reconfigure gdm
sudo service lightdm stop
sudo service gdm start
Adding my experience here for others. This happened again with 17.10 and after trying out various stuff I knew and told in the internet, it turned out to be xorg.conf issue. I replaced the /etc/X11/xorg.conf with an earlier /etc/X11/xorg*dist-upgrade*.conf version and it worked. I use Nvidia driver. So, I had to reinstall it. After that, I got my Unity on 17.10 back! Yes, I am still using Unity as opposed to Gnome-* on 17.10 for reasons I mentioned in my other answer.
Earlier, I had tried reinstalling lightdm, unity, unity-greeter, unity-session, removed gdm3, kodi, ubuntu-session, other nvidia driver versions, looking into .xsession-errors, /var/log/syslog, umpteen number of restarts selecting multiple kernel versions and what not. It took more than a day to fix this notorious and infamous login loop issue.