This morning I woke up to my phone using mobile data and my home assistant automations not working. Initially I thought it the power was out, but I could turn on the lights just fine. I checked my UniFi app and saw that the server was not connected to the network at all. This meant that the cable got unplugged, the switch isn't working, or the server isn't working. It said the switch was connected and another device was connected to the switch so that narrows it down to just 2 cases. So I opened my server closet in the basement and immediately noticed something was wrong. I couldn't figure out what was wrong but I just felt like something was wrong. Everything was plugged in, the network switch lights were blinking like normal, my raspberry pi was running just fine, even the server indicator lights were on. My main server is an old gaming PC so it has a glass side panel so I looked inside and I could see the fan spinning, but I could not hear it. Usually I have it set to full speed and I can hear full speed very well. I tried rebooting the server with the power button and the fans didn't go to full speed. As a last resort, I brought down a keyboard and monitor. As soon as I plugged in the monitor, I saw that there was a prompt to set the time on the BIOS! Picture of the prompt In my opinion, this was the stupidest reason for an outage.
Further investigations
I dug a little deeper and discovered that the BIOS had been reset during a power outage right before all of this happened. So far I have consulted the motherboard manual and found absolutely nothing about this. After a bit of research, I think it could have been that the CMOS battery has died. This is a really simple fix but I don't have the replacement battery right now. This means that I will have the same exact issue after the next power outage unless I replace the battery.
Preventing this in the future
From what I can see, I just need to replace the CMOS battery. But this computer has been running for over 4 years, so what is stopping this from happening again around 2028? The most effective solution is going to be preventing power outages in the first place. This can be done using a battery backup or a standby generator. Standby generators will last longer during a power outage but are typically more expensive and harder to setup than a simple battery backup.
This is the correct answer. No matter how reliable your power feed is you still need to reboot the server at some point for whatever reason and if CMOS battery is dead by then you'll have the very same issue and you'll need monitor and keyboard again. And even if you don't mind about the RTC on board you'll still lose the settings.
I wonder why manufacturers haven't switched over to supercapacitors or something else than a coin cell battery, but perhaps there's a valid reason for it. I think that supercaps can't hold charge as long as a coin cell, but if your board is completely cold for a year or so maybe losing bios settings isn't that big of a deal.
Well they still have a finite life and are less replaceable than a battery. Even if it quadrupled the lifespan (which is a reasonably generous estimate given OP's 4-year duration and wikipedia telling me supercapacitors last 10-15 years), it would still eventually need to be replaced and that would generally require resoldering it.
I think a much better solution is 2 battery slots, one to be a backup battery, unused, and then when needed, an LED on the mobo can be turned on. Honestly OP could jury-rig up a similar system if he wanted to, although it'd be a bit ugly and anytime something is jury-rigged I don't really think of it as reliable.
You could of course use some kind of socket or connetor for supercap, but as they last far less than I thought then I get why it doesn't make sense. This thinkstation I'm writing with in my garage I got for free at old office is from 2011 and it's still running original cmos battery. No idea if there's any juice left on it, but at least it doesn't complain anything at boot and once it refuses to boot it'll become e-waste immediately (I do metal working, fix cars etc at the garage, so internals of this thing are far from clean, I think this is 3rd or 4th hardware for 10 years in here with only the SSD moved from setup to another).