Is the baud rate in your program and serial monitor the same?
Be aware to the baud rate setting at the status bar.
It works for me! Thank you, @andxu.
Nice, add this issue to the FAQ of the wiki page.
Baud rate made it for me:
Ctrl + Shift + P
Arduino: Change baud rate
I choose 9600, I guess it is the default
The baud rate trick did it. Thanks for commenting.
Most helpful comment
Be aware to the baud rate setting at the status bar.