Show logs from the previous boot

-b -1 selects the previous boot and -n 200 shows its last 200 lines, which are the minutes before the restart or crash. There you can see, for example, the OOM killer, kernel errors or a regular shutdown. journalctl --list-boots lists all stored boots with their time range.

Note: Only works with a persistent journal. If the directory /var/log/journal is missing, journald keeps logs in RAM only and the logs of earlier boots are lost.