I suspect perhaps you used import existing project into workspace? The issue wouod be that you cant have more than 1 copy of the same model in a workspace. Eclipse has a lot of rope available to hang yourself :). The tool tries to watch for cases where multiple models with duplicate UUIDs are placed in the same workspace. If it sees this it will remove the xtuml nature from the duplicate project (2nd one added). With no xtuml nature there is no model to show in the xtuml perspective. It will warn you before it removes the nature though, you should have seen the warning if this is what happened.
I still dont think this explains every thing you wrote, but it sounds like it.
You could look at the error log view for more hints. Also, Im not sure what you mean by “fade mode”?