Arduino UNO flash problem

Hello,

I bought some original Arduino UNO rev3, and I have brick two of them. I change ATmega328P who work on over board: no change. I try to reset usb chip ATmega16U2, by a jumper on reset/GND, dit not work. One Arduino dit not reconize by windows ON led is on and L led is ON. The second one is reconize on windows but when I tried to flash I've got "avrdude: stk500_getsync(): not in sync: resp=0x00".

Some poeple have the same problem? Many people talk about "stk500_getsync(): not in sync" but I didn't find the solution Can I flash ATmega16U2 by ISP , How I can do that?

Thanks !