I guess the keyword is, "completed". If something is broken, I doubt it would be in master, or the team has done something terribly strange. Or they should be nicer to QA ;)
Rollback to last known good state. Hopefully that's not as far back as the previous release (been there, done that, all I got was this lousy egg on my face).
What happens when your software is in a broken state that's worse than what you had before?