System suddenly crashed, how to find out what happened

[alfwro]

I've been running proxmox on Intel Nuc (i7-10710U CPU) device for the past several years and it has always been rock solid. Today however it crashed - stopped responding to pings and I had to pull the power plug to restart it.

root@pve:~# pveversion -v
proxmox-ve: 8.2.0 (running kernel: 6.8.8-3-pve)
pve-manager: 8.2.4 (running version: 8.2.4/faa83925c9641325)
proxmox-kernel-helper: 8.1.0
pve-kernel-5.13: 7.1-9
proxmox-kernel-6.8: 6.8.8-3
proxmox-kernel-6.8.8-3-pve-signed: 6.8.8-3
proxmox-kernel-6.8.8-2-pve-signed: 6.8.8-2
proxmox-kernel-6.8.8-1-pve-signed: 6.8.8-1
proxmox-kernel-6.8.4-3-pve-signed: 6.8.4-3
proxmox-kernel-6.5.13-5-pve-signed: 6.5.13-5
proxmox-kernel-6.5: 6.5.13-5
pve-kernel-5.13.19-6-pve: 5.13.19-15
pve-kernel-5.13.19-2-pve: 5.13.19-4
ceph-fuse: 16.2.11+ds-2
corosync: 3.1.7-pve3
criu: 3.17.1-2
glusterfs-client: 10.3-5
ifupdown2: 3.2.0-1+pmx9
ksm-control-daemon: 1.5-1
libjs-extjs: 7.0.0-4
libknet1: 1.28-pve1
libproxmox-acme-perl: 1.5.1
libproxmox-backup-qemu0: 1.4.1
libproxmox-rs-perl: 0.3.3
libpve-access-control: 8.1.4
libpve-apiclient-perl: 3.3.2
libpve-cluster-api-perl: 8.0.7
libpve-cluster-perl: 8.0.7
libpve-common-perl: 8.2.1
libpve-guest-common-perl: 5.1.3
libpve-http-server-perl: 5.1.0
libpve-network-perl: 0.9.8
libpve-rs-perl: 0.8.9
libpve-storage-perl: 8.2.3
libspice-server1: 0.15.1-1
lvm2: 2.03.16-2
lxc-pve: 6.0.0-1
lxcfs: 6.0.0-pve2
novnc-pve: 1.4.0-3
proxmox-backup-client: 3.2.7-1
proxmox-backup-file-restore: 3.2.7-1
proxmox-firewall: 0.4.2
proxmox-kernel-helper: 8.1.0
proxmox-mail-forward: 0.2.3
proxmox-mini-journalreader: 1.4.0
proxmox-offline-mirror-helper: 0.6.6
proxmox-widget-toolkit: 4.2.3
pve-cluster: 8.0.7
pve-container: 5.1.12
pve-docs: 8.2.2
pve-edk2-firmware: 4.2023.08-4
pve-esxi-import-tools: 0.7.1
pve-firewall: 5.0.7
pve-firmware: 3.13-1
pve-ha-manager: 4.0.5
pve-i18n: 3.2.2
pve-qemu-kvm: 9.0.0-6
pve-xtermjs: 5.3.0-3
qemu-server: 8.2.2
smartmontools: 7.3-pve1
spiceterm: 3.3.0
swtpm: 0.8.0+pve1
vncterm: 1.8.0
zfsutils-linux: 2.2.4-pve1
root@pve:~#

here are bits from the journalctl before and after:

Jul 25 15:33:25 pve systemd[1]: Stopping user@0.service - User Manager for UID 0...
Jul 25 15:33:25 pve systemd[991896]: Activating special unit exit.target...
Jul 25 15:33:25 pve systemd[991896]: Stopped target default.target - Main User Target.
Jul 25 15:33:25 pve systemd[991896]: Stopped target basic.target - Basic System.
Jul 25 15:33:25 pve systemd[991896]: Stopped target paths.target - Paths.
Jul 25 15:33:25 pve systemd[991896]: Stopped target sockets.target - Sockets.
Jul 25 15:33:25 pve systemd[991896]: Stopped target timers.target - Timers.
Jul 25 15:33:25 pve systemd[991896]: Closed dbus.socket - D-Bus User Message Bus Socket.
Jul 25 15:33:25 pve systemd[991896]: Closed dirmngr.socket - GnuPG network certificate management daemon.
Jul 25 15:33:25 pve systemd[991896]: Closed gpg-agent-browser.socket - GnuPG cryptographic agent and passphrase cache (access for web browsers).
Jul 25 15:33:25 pve systemd[991896]: Closed gpg-agent-extra.socket - GnuPG cryptographic agent and passphrase cache (restricted).
Jul 25 15:33:25 pve systemd[991896]: Closed gpg-agent-ssh.socket - GnuPG cryptographic agent (ssh-agent emulation).
Jul 25 15:33:25 pve systemd[991896]: Closed gpg-agent.socket - GnuPG cryptographic agent and passphrase cache.
Jul 25 15:33:25 pve systemd[991896]: Removed slice app.slice - User Application Slice.
Jul 25 15:33:25 pve systemd[991896]: Reached target shutdown.target - Shutdown.
Jul 25 15:33:25 pve systemd[991896]: Finished systemd-exit.service - Exit the Session.
Jul 25 15:33:25 pve systemd[991896]: Reached target exit.target - Exit the Session.
Jul 25 15:33:25 pve systemd[1]: user@0.service: Deactivated successfully.
Jul 25 15:33:25 pve systemd[1]: Stopped user@0.service - User Manager for UID 0.
Jul 25 15:33:25 pve systemd[1]: Stopping user-runtime-dir@0.service - User Runtime Directory /run/user/0...
Jul 25 15:33:25 pve systemd[1]: run-user-0.mount: Deactivated successfully.
Jul 25 15:33:25 pve systemd[1]: user-runtime-dir@0.service: Deactivated successfully.
Jul 25 15:33:25 pve systemd[1]: Stopped user-runtime-dir@0.service - User Runtime Directory /run/user/0.
Jul 25 15:33:25 pve systemd[1]: Removed slice user-0.slice - User Slice of UID 0.
-- Reboot --
Jul 25 16:39:58 pve kernel: Linux version 6.8.8-3-pve (build@proxmox) (gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #1 SMP PREEMPT_DYNAMIC PMX 6.8.8-3 (2024-07-16T16:16Z) ()
Jul 25 16:39:58 pve kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-6.8.8-3-pve root=/dev/mapper/pve-root ro quiet intel_iommu=on
Jul 25 16:39:58 pve kernel: KERNEL supported cpus:
Jul 25 16:39:58 pve kernel:   Intel GenuineIntel
Jul 25 16:39:58 pve kernel:   AMD AuthenticAMD
Jul 25 16:39:58 pve kernel:   Hygon HygonGenuine
Jul 25 16:39:58 pve kernel:   Centaur CentaurHauls

I cannot see anything in the obvious log files (journalctl - no errors or faults etc)
How do I find out what caused it? Where else do I look?
Thanks