Have you ever been on a project when the patient goes “Code Blue”? It is crashing. No pulse. No breath. No blood pressure.
So what do you do? My first piece of advice is that you don't try to fix everything at once. On the battle field or in an emergency room they have the concept of triage. This means that you start with the most critical, life threating issue. In order to do that you must evaluate the patient.
List what the projects are for the project. Is there a lack of performance? Are there mysterious bugs that are driving the users insane? Possibly it is that the understanding of the requirements were wrong.
Like medicine, the most critical wound is if the heart stops. To an IT project the heart is the business need. If you can check of the requirements then move to the next step. This is usually bugs that are stopping the users from working.
Another key point is don't guess at solutions. Create your theories and test them out. Quick fixes often don't address the real problem and only prolong the bleeding.
Of course this doesn't cover everything, but it gives you an idea of an approach to triaging your project. Good luck.