After closing all apt processes (Synaptic, running apt-get/dpkg, ...) and running
sudo dpkg --configure -a
in a terminal as suggested, try
sudo apt-get update
and then retry what you were previously doing, for example
sudo apt-get upgrade
You can also try cleaning the cache if it's troublesome, though I don't think this is going to help much here:
When you run (for the first time in a moment) any command with 'sudo', you will be asked for your password. Type it carefully: nothing is printed on screen while you type. Like said, be careful when typing in the commands; if you for example leave off a space between two options (for example between '--configure' and '-a'), they're interpreted as one option which will probably fail. If you still don't get anywhere, you should copy-paste the error output here (except if it's the same as before: then you can just say so) so people can see where you're going. You should also mention if you've done (or tried to do) any modifications to your system that might affect the case at hand -- maybe changed some configuration files (of apt in this case), for example.
Apt can be a little picky if it's interrupted for some reason..hopefully you'll get this straightened out!