Arduino DUE : No device found on COM

Hi all,

When trying to upload a sketch, even "null" via the USB Programming Port of my Arduino DUE, via IDE 1.6.8, the following error message appears: No device found on COM (COM3 in my case) The RX lights about 1 second before the message is delivered.

I has worked till this week-end, but now, no possibility found to make the upload. I tried some erase / reset sequences but no results. I tried it on 3 computers, on Vista, Windows 7 and Windows 8 with same results. Also tried with mode com3:1200 ... Add change the USB wire...

Ideas of what happens ? And how to work ?

Thanks for tour help