Comment 3 for bug 1382285

I encountered the same problem and solved it. I have tried a number of things, so I am not 100% sure which one it is that solved it. If I were to do it all from scratch again, this is my suggestion for troubleshooting the problem:

1. look at /var/log/mysql/error.log for lines with the [ERROR] tag. In my case, the one preventing mysqld from starting (and thus breaking the upgrade which expected a running mysqld) was

  141109 10:48:23 [ERROR] /usr/sbin/mysqld: unknown variable 'default-character-set=utf8'

2. search for the meaning of the error message. In my case, I found <http://dev.mysql.com/doc/refman/5.5/en/server-options.html#option_mysqld_default-character-set>

3. edit /etc/mysql/my.ini to remove the source of the error. In my case, I changed the old charset and collation default options to

  # deprecated:
  # default-character-set=utf8
  # default-collation=utf8_unicode_ci
  # the newer options:
  character-set-server=utf8
  collation-server=utf8_unicode_ci

4. try again to start mysqld / run the upgrade and repeat until there are no new errors in the error log and mysqld is started after the upgrade.

  sudo apt-get upgrade
  sudo service mysql start

5. if necessary, try reinstalling (I did this, not sure it contributed to the solution, but just in case):
  sudo apt-get remove mysql-client* mysql-server*
  sudo apt-get install mysql-server