Well since i switched to ubuntu I have messed with synaptic a lot. This can come back to bite you in the ass some times. Things you need in your system like x11 shouldnt be messed with because they control a lot of things in the system. So with my experience of breaking and fixing my ubuntu system on about 5 or 6 occasions ill try to help others who find themselves in similar trouble. I use Gnome so I dont know if these will work with kubuntu.
1. Try figure out what the problem is (sounds simple but its the first step for a reason)
2. Reboot the system (It might just sort itself out)
3. If ubuntu desktop doesnt load try failsafe gnome or failsafe xterm
4. If its not booting try hitting esc on GRUB and going into recovery
5. Reinstall ubuntu (This is the most drastic but sometimes is the only way)
If it comes down to reinstalling ubuntu its not so bad stick in the ubuntu disk load up the live session and you can recover your data by putting it on a usb stick or by transfering the info to another computer(Using ssh).
Good commands for xterm are
sudo apt-get update (Get the latest list of updates)
sudo apt-get install (install the new updates)
sudo apt-get dist-upgrade
sudo aptitude install (package name) (This is good if packages are broken or dependencies are unsatisfiable)
Hope this helps someone it helped me a few times. If I left anything out leave a comment and ill add it.