Man, that was a PITA.

I decided in my ultimate wisdom to do some testing on my main rig... what a nightmare.

I was messing around with openvpn servers/clients and such, including upnp on my router to use plex outside of home... this was so I could help my friend doing stuff to his.

Anyway, so did all that, everything was fine, so went back to use my PIA vpn... and nope. So figuring it must have been the TAP adaptor, re-installed the adaptor and client... so OK now have internet access back. went onto my server IP, nothing!!!

So, check some more of my network stuff, Hue, etc... and nope. So go to router IP to check settings, and nope... switch off/on again... and nope

Pulled my server out... because it's network switch is behind it, power cycle it, still nothing... check other network switches and so on, same result.

I was pulling my f'ing hair out, I was like  "what the fuck, I didn't even touch the router - except for rectifying that DNS address I didn't recognise!!! - - - - -  then the lightbulb went on!!"

Sure enough, checked the gateway address, something I didn't think to check before as I hadn't touched that side of things, and sure enough the router's bloody address had changed just to reset a DNS address somehow??.... bear in mind this is like 2 hours after the problem appeared and my frustration reaching epic proportions... so anyway, put the router/gateway address back to what it should have been, checked DNS settings too while there, and success everything back to normal.

I still can't figure out how the IP address of the router changed, I know it needed to power cycle to change the DNS settings, but I swear to dog I never changed it, had to rhyme or reason to... and I don't allow remote access to my router, so couldn't be that.