I'm trying to upgrade to 13.10, and the upgrader/installer app has stopped. It doesn't seem to be frozen- it gives me a warning if I try to quit it, but it has been stuck like this for hours now.
The internet connection is fast and working. Am I going to break things horribly if I quit it and restart?
At least for my case, it turns out you can force a restart and then repair packages.
I followed @SauravKumar's advice in the comments and forced the system to restart. On reboot, the computer got to the purple screen where it selects which OS to run. If I chose the default Ubuntu, it started loading and then went to a black screen and didn't respond to the keyboard at all.
So I then booted in recovery mode and chose the check packages option. It found out that I was using a different version of etc/default/rcs than the package maintainer. I chose to update it (the default is don't update) and then resumed booting and now it's running fine. Just to make sure, I rebooted again, and it works after a regular reboot too.
Update to 13.10: success!