I see. Hope catymag help you also. If you want try my simple two advices:

1. Try connecting the device with USB cable while zumo is counting down. For example connect zumo with USB cable and after that disconnect it. Zumo should start counting down numbers to turn off. While counting down connect it with USB cable.

2. If that won't help try to reflash firmware or flash older version or if you want to take the risk try the cure firmware I have provided link in my previous posts.

Hope you succeed.

Cheers,