How do you approach troubleshooting a problem you have never seen before?

Describe a method, since IT interviews care far more about how you think than what you have memorised.

• Define the problem precisely first. What exactly happens, what was expected, when did it start, who is affected, and is it reproducible. "It does not work" is not a problem statement, and getting to a precise one solves a surprising number of issues immediately.
• Find out what changed. An update, a configuration change, a password expiry, a certificate renewal, or new hardware. Most things that used to work and now do not were changed by someone.
• Narrow the scope systematically. Is it one user or many, one machine or all, one application or everything, one site or the whole network? Each answer eliminates whole categories of cause.
• Work in layers. For anything network-related, work up from physical connectivity to IP, to DNS, to the application. Skipping to the complicated explanation before checking the simple one wastes hours.
• Change one thing at a time, and note what you tried.
• Search deliberately — the exact error text, and vendor documentation before forum answers.

Note: Saying you know when to escalate, and that you escalate with the diagnostic work already done rather than passing on a bare ticket, is a strong and practical point.