Root directory '/' locked after using chmod

As you can't use sudo, try the following command instead (you'll get root privileges with pkexec):

pkexec chmod 755 /

Tested ok in a VM:

u@u-VirtualBox:/$ sudo chmod 3 /
[sudo] password for u: 
u@u-VirtualBox:/$ ll
ls: cannot open directory .: Permission denied
u@u-VirtualBox:/$ sudo chmod 755 /
sudo: unable to stat /etc/sudoers: No such file or directory
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin
u@u-VirtualBox:/$ pkexec chmod 755 /
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/bin/chmod' as the super user
Authenticating as: u,,, (u)
Password: 
==== AUTHENTICATION COMPLETE ===
u@u-VirtualBox:/$ ll
total 104
drwxr-xr-x  23 root root  4096 oct.  12 20:22 ./
drwxr-xr-x  23 root root  4096 oct.  12 20:22 ../
[...]
u@u-VirtualBox:/$ 

First, try other suggestions while still logged in, because mine requires a reboot. If you see that no other attempt works, you should reboot and start a live DVD/USB session.

Mount the root partition and edit the /etc/fstab as root. You will need to run a command similar to this:

gksu gedit /media/ubuntu/sda1/etc/fstab

Please adjust the path to the correct mount point you used.

In the file, locate the line belonging to the root partition, it should look similar to the following:

UUID=[...] /               ext4    errors=remount-ro 0       1

Try to enforce "universal" permissions by adding the proper parameter:

UUID=[...] /               ext4    umask=000,errors=remount-ro 0       1

Reboot and try to log in again. You should be able to fix the permissions with:

sudo chmod 755 /

Warning

After the fix, remember to revert the changes made to /etc/fstab, otherwise anyone will be able to access and modify every system file.