That little bit of additional information is very important, as it marks a big difference between "bricked" (which means the bootloader is toast, and unable to even start booting) and "crashing" (which is your situation).
In your case, your router does begin its boot process since it goes far enough to start initializing the USB interface. That means that your bootloader is fine, the issue is either a corrupted firmware, or a corrupted nvram.
Usually, you should be able to use either recovery mode (by booting with the reset button pressed) or factory default resets (by booting with the WPS button pressed). However it's possible that the RT-AC68U might suffer from the same issue as the RT-AC56U, where corrupted nvram can prevent the router buttons from working at boot time.
In such a case, the router is always recoverable through the use of a TTL to Serial cable. No need to go thermonuclear with JTAG (which is far, far more complicated to do than recovery using a Serial to TTL cable).
You mention the USB led turning on - try booting your router with NOTHING plugged to it: no USB disk, no LAN, no WAN cable. See if it completes its boot sequence. If it does, now try pressing the reset button for 5 secs, until the power led flashes, then release it. That will do a factory default reset.
Thanks for explanation, at least it's not bricked, I guess it's more a corrupted nvram because the update process went fine, I had 100% flashing progress and the router rebooted it's self the first time, then I went in the webgui I could check that version was 378.50 in top of the screen.
All happened after I click on the factory default to reset it.
The problem now is that the router won't start in recovery mode, I try either reset and wps button, all unplugged, no way.
I don't have the "TTL to serial cable" so I guess I need to go for a service to Asus, it's still on warranty.
I already updated successfully this router 4 times now I'm just a bit afraid to upgrade the firmware in the future because this situation never happened before, also due to this nvram issue.