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?

...R