VESC stopped working, can't connect to USB

Hi,

I bought three months ago two VESC from DIYELECTRIC SKATEBOARD (https://diyelectricskateboard.com/products/torque-esc-vesc-bldc-electron...) 
to drive my drone motors. I flashed both of them with Vesc_Tool 0.87 and they worked very well and I was pleased with their performances.

However, about two weeks ago, one VESC stopped working and my PC (windows 7) isn't detecting it anymore when I connect it with USB. When I plug the battery,
only the blue led stay solid and the red and green led briefly blink each 3 seconds. I tried changing the cable, different batteries and using on different
PC (windows 10 and 7), but nothing different.

Just recently, the same thing happened to my second VESC and I just don't know why. I checked for possible hardware damages, but the voltage regulators
are perfectly working and by using a oscilloscope, I saw that the processor is clocking correctly.

I checked many forums, but usually, the VESC was just not working at the start and didn't jsut stop working after three months.

I'm looking forward for any advices or solutions for this problem.

Thanks