USB connection stopped working

MacBook Air, OS X 10.9.5
Arduino UNO, ATMEGA 328P
ESP13 wifi shield

I was trying to get my ESP13 to work together with the UNO. I had the ESP on top of the UNO and connected the UNO to the Mac with USB-wire. In the beginning the connection between the Mac and the UNO worked fine, no problem. After a while experimenting with the sketch suddenly the USB connection stopped showing up. I can’t see the USB-port neither in Arduino nor in the hardware-list in the Mac.

I think this happened after uploading a sketch where I had changed “(2, 3)” in this command to something else:
#include <SoftwareSerial.h>
SoftwareSerial EspSerial(2, 3);

I still can connect other UNOs to the Mac without problem, the USB-port shows up as it should.

You need to post your code so we can see what you can see.

Does the problem Uno work if you remove the shield?