You are not providing nearly enough information to help you. Saying "I have a problem - please fix it." is not productive. Tell us what your were doing when it stopped working, and what you did just before it stopped working, IN DETAIL.
no i did not make any new user..
i cannot install any software via the terminal i cannot add or view user and groups..and if i write sudo in terminal i get this error
sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner
sudo: fatal error, unable to load plugins
You can backup your user data with luckybackup, which is a GUI interface to rsync. Boot with a live ubuntu CD, install luckybackup, visit your hard drive with the file manager, plug in an external drive, and define a backup task in luckybackup.
For a faster way, use a tar cpvzf command to create a gunzipped version of your home folder.
Notice that I'm a beginner, as far as backup is concerned :)