Something killed the package manager while it was configuring the programs that it had installed.
This is most likely a power cut while it was running, but could also be caused by the computer running out of memory.
To fix the problem, the suggested solution is to tell the package manager to re-configure all installed programs. The easiest way to do this is to open a terminal (Applications->Accessories->Terminal) and then, in the terminal window, type this command:
sudo dpkg --configure -a
You need to start the command with sudo in order to get administrator rights to reconfigure the software on your computer. You will be asked for your login password (unless you've entered it into another sudo command recently).
It will take some time to complete and will produce some output to the terminal. When it's done you will be presented with a terminal prompt (should end in a $ sign before the cursor), and either a message indicating that it completed or possibly an error message. If you get an error, then please come back and tell us what the error is. In any case, try Synaptic after you run that and see if it works.
Hope that helps,
—Robert J Lee