So you have a showstopper. First is it even the right word. If you take one step back and look at the situation and all involved, its now the show begins.
Ok, late in the testing something is found. Your new version is not performing up to specs, you have something that is so big that you stand there with a showstopper, while the release date is getting near. The phones, emails and other communication devices are being heavily used as everyone tries to understand what it is, what went wrong, and how to avoid blame. There is the show, so it may be that a Show Stopper is a Show Starter.
It will however be much easier if you as a product manager take the lead and if you understand why the others behave as they do. Only then can you find the acceptable way forward and try to get the product out without the organization failing around you.
Project Management
Your project manager really wants to know what happened and what should be done different next time. Excellent questions, but try to convince him/her that these questions can be dealt with when the Show Stopper is dealt with. How to learn from this is not the primary task right now, it is to fix the mess. Get the project manager to look forward and help you find creative solutions.
Release Management
Who committed the release date in the organization? Right release management, so who want to do anything to keep it? Right again. But a delay may not be that fatal. That you delay the release to make sure that the quality is good is something that could be positive. Convince your release manager that the quality is the foremost, not the date. Take help from marketing if needed. Do whatever it takes to bring your release manager on board in getting the product out to the customers as soon as possible, but not before its worth delivering it.
Quality Assurance
It may be that QA found the show stopper, or it may be that they missed it. Now they will do anything to do all testing all over again when the error is fixed. But is that necessary? Involve QA in identifying the error and also assess how the fix is applied. It may be that if they get sufficiently involved they may find a better way. It may be that the can accept to just retest relevant parts. Help them get to that conclusion, involve them in the process. See them as your ally (really they are that). They have the same goal as you have, get out a product with GOOD quality.
Development
And of course, someone produced the bug. But really, right now is NOT the time to assign blame. Identify who is the best to fix it, work in teams if needed. Make sure the fix is solid and well documented (you need that when dealing with QA). Then later on when the product is released (on time or later) its time to ask the question “what can we do different next time”
Sales and Delivery
When trying to get forward after an abrupt halt make sure that sales and delivery is not hurting to much. They have tight schedules, they are the ones getting the reactions from the customers, and they know if there are any contractual obligations or not. In short, they provide you with the information you need to have when deciding if work is around the clock or not. Just make sure you differentiate between MUST and NICE. If they say it MUST be released before weekend, ask WHY, ask about contractual obligations and project plans. Make sure they don’t pull the must card once to often.
Conclusion
Cooperation and a clear target are the two most important things. You as the product manager are in a unique situation to get everyone else to cooperate. Make sure you listen to them, factor in their input and then get everyone to strive for the same goal.