"Give me an IP, username and password - what OS is it?" are about all you start with and go from there. It's probably a critical system to someone, and everyone swears on a stack of bibles that nobody did anything, touched anything or made a change. You have very specific domain knowledge (kernel, grub, SAN/storage, systemd, dbus, etc.) and typically ask a lot of questions to the systems owner as your fingers are flying ruling out reasons (low hanging fruit common issues).