COMM port not found

Hey Arduino Community,
I've had my first arduino (uno) for a couple of months now, and its been great. I haven't had any problems with programming, uploading, or using the hardware. However, I was working on a project on a breadboard that received a signal from an IR receiver and subsequently turned on a motor via a transistor. It worked fine on the breadboard, but then I actually started soldering the components and now the program doesn't really run (the indicator LED for the program works). So, I tried to hook it up the serial monitor via USB and the IDE didn't find it. Whenever I connect it, my Mac normally says it found a new network interface, but now it doesn't anymore. I'm afraid some solder fell and shorted something out on the board, but it looks fine. Any ideas?
Thanks

I think it might be a problem with the new Atmega8u2 USB chip. I now have 2 Mega 2560 (they use the same usb-chip as the Uno) and both aren't visible on the computer anymore. The first I thought I burned it because the power supply had become very hot, but the second one just stopped out of nothing and completely shut down my computer when it happened. Program and everything was running fine and the second Mega hadn't become hot.

Does your still function except for the communication to the computer? If so I think we have the same problem, my topic is here.