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 ??