I originally came here to ask a question but I ended up resolving it about 10 minutes ago. So instead I’ll share what happened to me in case others are in the same boat.
Earlier this week my printer (Octopus with RPi host) shut down due to a power failure, and when I turned it back on, I found that the Mainsail interface at <hostname>.local was no longer running (Page Not Found). It wasn’t accessible via the IP address either.
My first suspicion was network issues, but I was able to SSH into the machine and ping the gateway / router / several laptops and nothing worked. And when I checked to see that Moonraker was running properly, I saw no warnings at <printer_ip>:7125. And my HomeAssistant plugin was having no problem running macros from my home automation system (lights, temp sensors, calibration, etc) as well.
Then I worried that maybe some sort of update had borked my Mainsail installation, so I did a wipe and reinstallation using KIAUH, but still nothing. But it did get me thinking about why the web interface would fail even though the Moonraker API wasn’t throwing errors. So I started sniffing around in the logs, and that’s when I saw issues with NGINX.
Lo and behold, it was failing to start the service because it couldn’t find /var/log/nginx/access.log. And upon further inspection, the nginx directory didn’t exist at all.
So I created that directory, and ran sudo chmod -R 755 /var/log/nginx to give processes access to it. That did the trick, and after a restart I’m back up and running.
Happy printing, y’all.