[motxilo]

If you knew what the problem was before the outage happened, and didn't consciously put the effort to prevent it, then oh well, that's professional negligence. Maybe not on you because you may not have the decision power to apply large-scale changes, but definitely somebody within the company. And if you didn't know what caused the outage, and you conducted some type of investigation to get at the heart of the problem, then you need a postmortem.

That's one of the main goals of a postmortem: document what the cause of the problem was, not just for you, but for all interested parties. If some human caused the issue, you need to document what happened and what should be done to prevent it from re-occurring. And fix it, of course.