Arduino Uno not showing on OSX (10.9.2) anymore

Hi everybody,
Here is the problem:
I got my board couple of days ago (Arduino Uno rev3), downloaded the IDE and uploaded the Blink example and everything went fine. After learning some basis I got my SeeedStudio NFC Shield v2.0 plugged on the board, uploaded one of the example provided with the library (PN532 e NDEF) and nothing works anymore!!!
The arduino doesn't show up on the port list of the IDE anymore, the power led is on but the board can't connect to my mac...
Is the board completely gone?
Is there some test I can run to check if I can recover it?
Any help will be very very appreciated!!!
Thank in advance for any info you provide

Davide