Serial Port not showing up when I plug in Arduino Mega 2560

So I've been using my arduino mega for a robotics project without problems for several weeks now. I plug it in, select Arduino Mega under Board, select ttyusb (something or another) under Serial Port, and code away. Now, for no perceptible reason, I plug it into my laptop and the correct serial port just doesn't show up. I plugged my friends Arduino Mega in (a lightly different ADK version, but nevertheless) and sure enough the ttyusb serial port showed up, I selected it, and programmed to the board just fine. Is my board trash? Is there some way to fix it? Has this happened to any one before? Thanks for your help.

I am on a Mac. The code I had previously loaded on still works and runs the robot, I just can't edit it because the board will not show up. The power light comes on when I plug the USB into my computer. The lights are on but the door is locked, if you will. My friend plugs his Arduino Mega into his Windows laptop and it shows up under device manager. He plugs mine in and it does not.