Although this may sound simplistic, whenever I was asked to turn around troubled projects (which was on several occasions) I just let three questions quide me:
1 Where are we?
2 Where do we need to be?
3 What do we need to do to get there?
Occasionally I consider a fourth question:
4 How did we get here?
But I usually set aside that root cause analysis for a later time. I try and stay very focused on how we should move forward rather than considering what happened in the past. And I try to get the team, management, customers, suppliers, all focused on moving forward.
Once you start to get these three questions answered, you can then focus on all the other activities to get buyin, key decisions made, plans set into motion, etc.
Also look for some short term deliverables that can demonstrate to everyone that the team is able to produce. This is a good confidence booster both for the project team and the external project stakeholders.
Good luck!