Do a post-mortem, work out root causes, work as a unit to ensure this doesn't happen again.
Obviously if there are levels of gross negligence or misconduct discovered during post-mortem, that will need to be dealt with accordingly, but coming into this with an attitude of "we must find someone to blame and incur repercussions" isn't healthy at all.
We are humans - don't forget that.
edit: forgot some words.