You have probably realized this in your first week on the job - security professionals are not well-renowned for the quality of their sleep, or the health of their livers. This is a guide for everyone who is dreading the day when the excrement impacts the oscillator. I'm not going to tell you about dealing with a breach in a technical or legal sense; I'm going to talk about maintaining your mental health and career prospects during one.
Before anything else, no matter what field you work in during times of crisis you will see everyone's true colors brought forth - not least of which - your own. You will know more about yourself and your co-workers after the event than you ever did before.
It comes as no surprise that Murphy's Law will likely bring itself to the fore (true to its nature) at the very worst time. That one web proxy that isn't yet configured to log to the SIEM - that will be the one your attacker filtrates your data through. That virtual machine cluster that was provisioned a month ago, but is not yet in active use, is where they will stage the attack from. That four-hour quarterly maintenance window that went badly and you lost all the logs for - that is when the breach will happen.
You are going to have your deductive skills tested to the limit; breaches happen through the places you were not looking. If you are lucky, you will be able to infer what happened through the remaining audit artifacts on your network. You are going to have to make large leaps of deduction, and justify them to people desperate to hear "at least they didn't take everything!” and “It’s not as bad as we thought!"
During a breach, you will find a whole chain of people that previously were merely names on an Org Chart become imminently real. If your experience at the job has been constrained to sitting quietly at your desk doing 'your thing', you are going to have more exposure to the executive leadership of your enterprise than you ever imagined. They are going to require fast and decisive answers from you - welcome to their world - you will be asked to make quick assessments of the information you have available and be held accountable for them afterwards.
You will perceive everything you've spent your time on amount to nothing, you will rant and rave to yourself (and all listeners) that this is just the proof you should quit and find another job. Take solace in the realization that things may have been much worse without the work you rendered. It is not that a breach occurred; it is the scale of the breach that matters.
Remember that business endures, for better or worse. Realize that the truth of what you saw will never see the light of day - it will be spun into an acceptable story and you will be bound by law to keep the secrets of someone else's failure. The real trick is to survive the process with your sanity intact.
Your first responsibility will be to create a complete and detailed timeline. Your job now is to discover and document how this happened - but not your interpretation of why this happened - as much as you want to invoke all your "I told you so!" instincts, this is not the time. A complete blow-by-blow timeline of how everything happened within your network is the primary information your command chain needs of you.