We recently upgraded to ArcGIS Server 10.6 and I was able to publish image services that display beautifully. However, any image service that was published on an older version of Server (10.4) are not displaying at all. We have hundreds of image services that are inaccessible right now and it is most frustrating.
We have not heard this as an issue. Can you provide more information. Can you check that the mosaic datasets (or individual rasters datasets) open up correctly in ArcGIS Desktop.When you say they are not displaying at all, can you confirm if the services appear in the service directory or not. If you publish a previous mosaic dataset can you check the Analyzer provided as part of the image service publishing states. Are you publishing to Image Server or are you publishing to ArcGIS Enterprise?