Try with other cables, as the USB Cable exist in version only to provide power/5V and not data. It could be that you use a wrong cable. Try also with another computer that is like Win7 or WinXP, try another USB Port of your computer as well.

As johnny told, when you insert the USB Cable into the PC (connected to the box) or vice-versa, it should detect the box (also if the Dreambox is not powered on).

Which image did you uploaded into the box (for which sim) that got your box dead ? As for sure just inserting the sim for that image, your box could get back to life. Then you can flash back a compatible image for your old sim if you want or continue to use the sim of the current loaded image. This is often the way used if the RS232/Service port is no more active in a NO CA ERROR case as this one seems to be.

TS