After flashing the Jetson TX2 with ConnectTech, nothing appear on the monitor
You would have to use the Connect Tech board support package. The default flash software would probably have at least parts of the Jetson failing to work.
Also, the monitor must be true HDMI. Are you using any kind of VGA-to-HDMI adapter? This would fail. You might want to describe the monitor and cable in more detail.
It isn’t unusual for the video to fail, and yet have the rest of the system running correctly. The best method to actually know if the system failed is via serial console (which has no dependency on monitor, ssh accounts, so on). For information on serial console, see:
Serial console also has the advantage that it can log the boot directly to the host PC. If you have a boot log, and post it to the forums, then a lot more can be examined.