Datasheet

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program.

If you crash your user program, or have a program that halts or otherwise fails, the user COM port can disappear.

When the user COM port disappears, Arduino will not be able to automatically start the bootloader and upload

new software.

So you will need to help it by performing the click-during upload procedure to re-start the bootloader, and upload

something that is known working like "Blink"

I don't understand why the COM port disappears, this does not happen on my Arduino UNO!

UNO-type Arduinos have a

seperate

serial port chip (aka "FTDI chip" or "Prolific PL2303" etc etc) which handles all

serial port capability seperately than the main chip. This way if the main chip fails, you can always use the COM port.