It's going to happen. That dreaded moment when the world falls apart. There's a big bug. And now is NOT a good time to have a big bug. Now what?
Never let 'em see you sweat.
You're the QA engineer. You're supposed to know this system inside and out - what it does, what it ought to do, and how to work with it. In the end, QA knows the system as a whole better than almost anyone. So when there's a problem, in the end it falls to you to figure it out, or at least to figure out how to figure it out. Marshall development, support, whatever you need, but you're the go-to guy right now.
So in a moment where everyone around you is panicking because something has gone wrong and needs to be corrected yesterday, that is the moment you must not panic. Be calm, be level-headed, and think your way through it. Being calm and in control at this point greatly increases your chances of success.
You can do this.
The second rule is this:
Let 'em know you take it seriously.
It's possible to take calm too far. Sure, panicking is bad, but it's just as bad to be perceived as not taking the situation seriously. Making a joke of the problem or putting it below your other priorities - even only in other people's minds - will make them think you're not engaged. This only increases their panic. Be engaged, be on top of the situation at all times. Work the problem and work it publicly. Just don't lose your head.
You can do this, too.