... after having done so for years!! Whether I click on the icon in the tray or double click the icon on the desktop, the icon glows for a moment, then goes out. Cold start does nothing. Worked fine last night and I haven't installed s/w of any kind (not for a week, anyways)

Have you verified that it's not running minimized as an icon in the lower-right corner of the tray? And that there is no mirc in task manager's processes tab?