arduino uno serial port not found

i had uploaded the blink program to my arduino uno from my mac and it work fine but when i went back to my computer to try to upload another program the serial port was not in the tools menu. i tried unplugging, restarting the IDE and resetting but none of these have worked.

Have you checked your device manager to see if anything shows up under libusb-win32 devices? If it does, you need to use FLIP and program the device, but it doesn't make sense unless you overwrote the ATMega8/16 with something that doesn't run the serial port application.