Professor and Director of the Playful Learning Lab AnnMarie Thomas wrote a column for the Star Tribune on the importance of constructive feedback after a failed project in the workplace.
From the column: The first thing I advise is to acknowledge that all companies and leaders face setbacks. As a leader, your team is going to be looking to you for guidance as you figure out what to do next. It is important that you recognize the hard work that inevitably went into the project. Your team spent time and energy on it, and should be given a chance to discuss what worked and what didn't. There are surely some lessons that can be learned from the experience.
I've also never encountered a project where there weren't at least a few things that went well. Start with those. A roundtable discussion where everyone on the team is asked for a positive insight from the work is often enlightening. (No win is too small. Perhaps the late nights led to finding a new favorite local restaurant, or a team member learning to use a new type of software.)