If rabbitmq cannot start and give you this following error.

BOOT FAILED
===========

Error description:
   {could_not_start,rabbit,
                    {previous_upgrade_failed,{rabbit,start,[normal,[]]}}}

Then, there is something wrong in your queue messages. The easiest fix would be deleting all the existing queues.

$ sudo rm -rvf /var/lib/rabbitmq/mnesia/*

or on OSX (via brew)

$ rm -rvf /usr/local/var/lib/rabbitmq/mnesia/*

Start rabbitmq again :) it should run fine.