What I did
apt-get purge python-qt and then
apt-get install python-qt as I had some problems with it (while installing Ninja IDE). When I logged out and in, Apport came in saying "Error report: ubuntu-one-control-panel-qt` is missing" or something like that.
What happens now
Now the Volume slider in the notification are is disabled/not working. I can't change the volume using my keyboard either (it has Fn shortcuts as it's a laptop). What should I do?
What I tried
I opened Synaptic and saw that the package Apport mentioned was not installed. I installed it, but even after restarting, that didn't resolve the problem. Google didn't help either.