USB problems with NANO

After i restarted my notebook the NANO isnt reqonized anymore. Is this a known problem and how kan i solve it?

What operating system are you using?

Have you checked to see if the Nano was assigned a new port/name?

I am using Windows7 and no there was no port assigned. The message when the device is plugged in that it is not requeonized.

[edit] Using a FDTI usb cable hooked to the RX/TX connections i could upload a sketch to the NANO, so the processor chip is OK. It seems that i have fried the USB chip somehow, strange. But reading some more contrubutions here its not unlikly. [/edit]