How should I find reason for a sudden break down?

The system I currently using is a headless machine using the Tiger vncRecently I have been using the system to run some algorithms like the xgboost, however, the system frequently disconnect, the frame of the VNC just stuck there, and the ssh give no response and cannot be reconnect.

You could examine the logs on the remote machine ─ with journalctl and dmesg ─ but chances are that they won’t contain anything useful regarding the crashes, because when the system crashes, it also stops writing to the logs.