The problem started in the following way. I was using tasksel from the terminal menu that it shows when I simply do sudo tasksel. I select Lamp Server from the list and started to install all packages related to it. The problem is that by mistake my friend clicked on the termina close button and then on accept, closing the terminal and the activity tasksel was doing. I tried to install the lamp server again and found the following problem:

sudo tasksel install lamp-server  

tasksel: aptitude failed (100)

I should mention that I already have aptitude installed. So the error is not that I am missing aptitude. It worked before closing the terminal. How can I solve this?