There are two commands that are worth remembering for this situation
when it arises. Open up a terminal window ( Applications / Accessories
or type Terminal into the Dash if using Unity ) and run the following
commands one after the other ...

sudo dpkg --configure -a

sudo apt-get install -f

You'll be asked for your password but when you type nothing will
appear on screen - this is normal and all part of Linux security it
inherited from Unix, so just type blind and press the [Enter] key.