I tried upgrading Ubuntu from 12.0 to 14.0 and the upgrade failed. However, when I tried to start apache2 afterwards, it fails with the message:
*Starting web server >apache2 * * The apache2 configtest failed. Output of config test was: AH00534: apache2: Configuration error: No MPM loaded. Action 'configtest' failed. The Apache error log may have more information.
But apache2/error.log does not have any entry pertaining to this failure.