I think that Ubuntu is not stable at all for a beginner or a normal user. All the time there are errors and problems, bugs need to be fixed and reports sent.
Now .. when I tried to manipulate the Unity interface .. things went wrong .. and I used the reset command as follows :
unity -- reset
But it never really reset. The terminal showed that the computer is doing something but I left the computer to complete this reset for more than 12 hours and it didn't finish .. and the error messages appeared in the terminal were so many. "Warn : cannot fetch the children of ..... " and things like that.
Soon, another strange problem appeared. When I click on Shut Down .. it just log out !! and when I click on shut down from the log in screen .. the screen disappears for a second and it comes again. I had to press the power button from my PC in those situations. The strangest thing is that it doesn't happen all the time, bu it happened twice or three times till now.
So .. backing to the main question .. If I need to reset or repair the whole system (Ubuntu 12.4 LTS) but I don't have the installing material because I upgraded from Ubuntu 11 to Ubuntu 12.4 LTS online .. how can I do that ?!