Arduino dont show on device manager, Show as Unkown device on DFU Mode

My arduino dont work more for upload sketches. the arduino is simply not recognisied on the system. it power up and run the latest sketch upoladed, but it doenst show on device manager. when i put it on DFU, it is detected but as a unkown devicie on USB. i tried to reflash the 8u2 using atmel FLIP, but i cant.

Thanks for help.

You need to install the drivers. Take a look here: http://forums.adafruit.com/viewtopic.php?f=25&t=19751