At one point this evening I completely lost the LAN. All wireless connections went down and even on a direct wired connection I could not ping the router.
All LEDs looked normal.
My connected modem showed that the internet connection was not affected.
I hard rebooted the router and all came back normal with full access.
When I went to look at the System Log I found the previous entry to the reboot was showing as
Sun Jun 21 21:19:55 2020
The next entry after this was following the reboot.
Is it normal that I was getting no entries whatsoever into the System log for 10 days?
Is this likely to be related to the source of the crash?
In addition to what @fantom-x said, the log is stored in RAM. As a result, the log is cleared by a reboot, so you won't see anything prior to the reboot.
If you experience crashes, with any real frequency or consistency, you should certainly look deeper in to what is going on -- examine your config files and the like. If you think the the logs may contain the critical information, setup a syslog server on another device on your network to send the logs from OpenWrt to an external server. Or, since you have USB on your router, you can write the logs to a USB drive (warning: constant writes are not good for flash memory, but OTOH, logs aren't too big or taxing and USB sticks are cheap).
Not saying that flash memory wears off quickly, but just keep in mind, that adding one 30 char line (or even a single char) requires a re-write of a 4K block. The next line will re-write the same block until the block is used up.
@fantom-x - true. I didn't really think about the data size mapping to the block size. That said, as long as the flash storage is cheap enough and physically replaceable (such as a usb stick), it should be of little concern if/when the flash starts to degrade (even if not ideal).