I would suggest cleaning out the list file cache as well as look at the sources like bashing-om suggested.
also clean the package cache
sudo rm -rf /var/lib/apt/lists/*
If you run it again and it freezes, try to get to a console.
Press ctrl + alt + F1 at the same time. If this gets you to a console, login with your user name and pasword.
You can then kill update-manager with
sudo killall update-manager
If, after these two commands, it still wont die then use this.
sudo kill $(pgrep update-manager)
You should try one of the first two commands first and check to see if it is dead by going to to the graphical display.
sudo kill -9 $(pgrep update-manager)
You can get back to the graphicak display by pressing ctrl + alt + F7 at the same time.
Only use the last command (SIGKILL) as a lsat resort.
If it does freeze after your sources have been checked and the caches cleaned then please post back stating if you could get to a console and kill update-manager that way and also state if you had to use a SIGKILL to kill it.