Double clicking the MKR reset button has to be done quite quickly.
Device manager will then show it as a different COM port with the word BOOTLOADER inserted.
Not too much to go on here but lets see about a few basics.
OS ?
Type of USB port 2.0 or 3.0 ?
Have you tried other cables as some of them are CHARGE only and not DATA and CHARGE ?
Have you ensured you have removed anything connected to the Arduino ?