Your question is Troubleshooting a Mission-Critical Failure. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How do you approach troubleshooting a mission-critical system failure?
Keep your answer focused on your first 15 to 30 minutes, how you separate symptoms from root cause, and how you decide what to check first when the system is down. Assume a production incident with active user impact and no time to guess.