...and you're on IIS and you have Windows Updates managed "for you". There was known quality issue with a Windows Update that blows up IIS: It can't restart. As upgrading ArcGIS Server requires a reinstall of the Web Adaptor, which requires an IIS restart, you're stuff out of luck if you did try it. Fortunately, there is a sort of fix.
See these threads for lots of troubleshooting and speculation:
My 2 cents, it'd be great if the WA install had some pre-install testing that could be run, so if any IIS stability issues are detected, it would recommend not uninstalling your current WA. 6 hours of my life I'll never get back, thanks M$........
Thanks for sharing. Our production systems are still 10.5.x, but we do have 10.6 in development and this may impact our upgrades there. I see lots of references to 2012 R2. We run 2016, so I will have to dig deeper to see if that version is impacted.
Yes, I foolishly uninstalled the 10.5.1 WA and attempted to install the 10.6.1 WA without checking to see if IIS would restart, something I know to check. Installing https://support.microsoft.com/en-us/help/4338831/july172018kb4338831osbuildpreviewofmonthlyrollup allowed me to proceed with the install. This was on production, not development. I'm so used to ArcGIS server upgrade working flawlessly...but there is a complicated arrangement of Portals and Wed Adaptors on our dev so.....take away my parking spot!