As mentioned by Gerald, Ubuntu 16.04 is out of date and should be upgraded (or get a new server with 20.04 and turn off the old one).
After 6 months of constant running, a small glitch that forces a reboot is not unlikely. There are several reasons for such, and what you have looked at could have given you a clue, but when you see absolutely nothing there are two main reason why it could happen:
- Power went out
- Hardware failure that could not be detected in software and the server has a watchdog allowing it to at least shutdown (maybe it sent an APCI message although that could have been registered, if the hardware is not functioning quite right, the software would fail reporting the issue)
If it did not auto-reboot, it was just stuck, that also points to a hardware issue and the software never got a chance of doing anything about it.
The /proc/sys/kernel/panic file is a flag to tell the kernel what to do on an error detected by the kernel (i.e. in software). You can find details here. Since you say it is zero, it would not auto-reboot in software on a panic.
On my end, I once had an issue with a Bluetooth receiver. It would receive so many hits that after a few minutes, it would create a lock up in the kernel. However, it was not clearly report in the logs. That computer, however, had a debug mode and I could listen for messages on the USB port. That gave me the info I need (tons of messages about Bluetooth). Turning that off made the system stable again. Since you had just one event, I would not worry about it. If it repeats over and over again, it's probably time to consider using different hardware.
