I uploaded the blink example code to nRF52840, but then the microcontroller doesn’t connect to my computer anymore. We can see the blink example working, but my computer doesn’t recognize it in any COM Port even though I double pressed the reset button.
It depends on how your code is written, but this is how it works, it is a little frustrating for sure, till you get used to it, but just be patient
you may need to use a “double click” motion on the reset button
Also some limited success by reducing serial com speed in your sketch to 19200 or below
Q2: My board is not showing up as a serial device on Arduino IDE
You can first try to reset the board by clicking the “Reset Button” once. If that does not work, rapidly click it twice to enter bootloader mode.
Hi there,
Some times it takes a few tries to get it to go into boot mode with a sketch that thin.
the cadence for the press is like morse code, NOT pressing it too quickly like DOT,DOT more like,
plug in USB and press it like Daash, Daaash, and be intentional about it. and do it more than 3 times if need be, The window will pop open and you’ll see the drive contents… Now your there. 
Go drag the RGB test Uf2 file in this thread it is in a ZIP file and drop it on the open file explorer drive.(Bootloader mode)
Read that thread too, it will help you. Sometimes the older example codes Nurf-UP(technical term) the Bootloader or code space or if you wrote garbage code to it , so this fixes that in a snap. and it just works again , even without using the boot loader mode now.
Note, the IDE may see it as a different com port so Know that too. YMMV
HTH
GL 
PJ
