BMW Motorrad Navigator IV and zumo 660 are identical internal hardware (they have the same HWID of 0924). Your info implies that the Nav IV has a hardware rather than a software problem - i.e. Symptoms of unable to copy files to computer in Mass Storage Mode and its GarminDevice.xml file having no content, the latter means the device is incapable of updating itself on boot cycle (that's likely why it was so slow to start-up previously).

What exactly do you mean by "Now it is not working any more"? If it appears completely dead but you are sure the battery has charge, take it into a very dark room and hold the power button to see if the screen glows slightly.