This is on Windows 10. Upon startup, Spikey apears, and the last thing he says is: "Initializing Default Notebooks." When I add "-cleanstart" to cmd line (properties of Mma shortcut), he says "Starting with clean preferences . . . " but still there's no front end. The kernel remains available. Much obliged.
Bruce at Wolfram Tech support to the rescue. His suggestion works for 12.0 as well:
Bruce - I really appreciate your help.