You have changed the permissions of every file in the filesystem. Among other things running
chmod 755 on a file clears the
setuid flag (any executable with the
setuid flag set runs as the user that is the owner of the file), and as this is used by a few executables that are part of the system (such as
sudo, which is owned by
root and relies on being run as
root when any user runs it to be able to work), clearing this flag means that they won't work. Also, the
/tmp directory (for temporary files) has
777 permissions (anyone can write) so that anyone can put files there. Removing that permission will break things.
These are just a couple of things that you broke when running
sudo chmod 775 /. As for fixing this, the easiest solution will be a fresh install of Ubuntu. System files have the permissions they have for a reason, and changing them can break things and introduce security risks. If you have anything you want to keep and can still use the file manager, copy them to an external drive. If not, boot from the Ubuntu Live Installer CD/USB and use that to copy your files off. Then check that your files actually are on the external drive and intact and if so then reinstall Ubuntu on the computer. And in future, don't make recursive changes to file permissions like that and remember that
sudo lets you easily wreck your system if you don't know what you're doing.