...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.
- Microsoft Update Catalog
See these threads for lots of troubleshooting and speculation:
- July 2018 update KB4338815 and KB4338824 causes issues within World Wide Web Publishing Service on Windows Server 2012 R…
- IISReset Issues after July 2018 Patches
- IISRESET results in W3SVC stuck in stopping status after July 2018 patches : The Official Microsoft IIS Forums
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$........