Mark,
Thanks, I was able to get GeoEvent to start up again without having to do the steps you outlined, but I will do those as well as it should clear up that error.
Here is what worked for me:
This fix online pointed in the right direction, but was outdated. Many of the files are in new locations for 10.5.1.
https://esriaustraliatechblog.wordpress.com/2014/08/12/geoevent-processor-wont-start-up-help/
Shutdown ArcGIS Geoevent Server service from Services panel. This would just hang and eventually time out. Looked at the following file to get the PID of the GeoEvent service:
C:\Program Files\ArcGIS\Server\GeoEvent\instance\instance.properties
From taskmanager, clicked Details tab, find the java.exe with that PID and deleted it.
From services panel, shut down ArcGIS Server service
Deleted the entire data folder at:
C:\Program Files\ArcGIS\Server\GeoEvent\data
Deleted the Zookeeper folder at:
C:\arcgisserver\local\zookeeper
Startup ArcGIS Server service in Services - and waited for about 5 minutes or so for it to complete load.
Startup ArcGIS Geoevent server service
Waited a few minutes and it login page loaded ok.
The fixes were all there in the forums, spread out a bit, and outdated with the locations of zookeeper.