Clear a dpkg lock
Occasionally after a problem, a dpkg lock gets left in place even after Synaptic and apt or aptitude are closed. At that point apt/aptitude/synaptic will complain about the d[kg lock and quit.
This usually happens when a program like Synaptic or Update Manager gets an error and quits without clearing the lock. After you have made sure none of the package managers are still running try this:
sudo dpkg --clear-avail