USB port not recognized

My fellow Arduinomaniacs
First forgive me if this topic is in wrong subject.
I have a serious problem because my project is for my master degree and I must present it until the end of the month.
So my project is a "genius sun tracker" with arduino.
Everything was fine until yesterday when I left the machine to collect data that serial sent to pc for monitoring.
After 15' I went to check but I saw that my program was stuck and the serial port wasn't recognized. After some attempts to pull-put the usb cable, to reload the USB drivers nothing! Still the pc doesn't recognize the arduino. I have another board that works so the problem is not from the pc.
I am afraid that the back EMF from the motor done the dirty work. I use this H-bridge to drive them with PWM adjusting speed.

Another thing that came to my mind is that I also use Softwareserial with the same BR 9600 with the hardware serial for remote guidance. Is possible to have some sort of conflict and had this problem?
Any suggestions ??
Thanks.