nRF52840 preview development kit booting error

Hi, I'm writing this because of nRF52840 booting error

I got this product today and It worked well, but after I downloaded one of the example's ~DEBUG.jlink file to the kit, I could not reach to the kit via PC anymore

The jlink file was "proximity example file" and my phone still can detect the nRF52840 kit and connect to it (and works very well)

HOWEVER, the PC could not reach to the board and so that could not download or use other programs

Could anyone help me??? I tried 

https://devzone.nordicsemi.com/f/nordic-q-a/24592/my-nrf52-nrf51-dk-kit-doesn-t-work

and 

https://devzone.nordicsemi.com/f/nordic-q-a/25794/nrf52840---not-detected-on-win10-pc---anymore/101583#101583

but any of them works for my case

Thank you

ej Park.