First try rebooting.
Then you can try this..(note this is probably NOT what you are supposed to do EVER.) This can break apt(and has before)... but this is what I do. and before you type this command you should wait for someone else to explain the possible problems from doing this.
i check and make absolute sure nothing is using apt. (another terminal window, synaptic, ubuntu software center) and then if it still says that.
$ rm /var/cache/apt/archives/lock
most of the time it fixes the problem without anything detrimental as far as I can tell (AS LONG AS YOU MAKE ABSOLUTE SURE NOTHING ELSE IS USING APT). It could have been caused by accidentally closing a terminal while it was installing something or something.