Sometimes the Arduino IDE loses the serial port. You had it working earlier. Suddenly you can't connect to your Arduino anymore. You have already tried disconnecting the USB cable, tried another USB port, restarted the Uno, restarted the computer etc. This might happen if you work on a Uno, switch to some other type of board for a while, and then go back to Uno.
I'm using two methods for this problem:
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Powered USB hub
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External power for the Arduino board
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It seems the name of the serial port gets "banned" in Mac OS somehow. Arduino IDE can't see it, CoolTerm or other apps can't see it either. But in "About this computer" -> System info -> USB -> the Arduino is listed as a "communication device". It's clearly connected and recognised by Mac OS. What usually works for me is to force the Arduino port to use a new name: I use a (powered) USB hub for this. Once connected via the hub, the Arduino shows up again (under a new name).
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Sometimes I temporarily connect a lot of peripheral ICs to the Arduino. It seems that if the load is to high, it can affect the connection to the IDE. The Arduino is still present in "About this computer" but not in the Arduino IDE or other apps. So I connect an external 12v power source to the Arduino Uno board.
Yes, I'm aware that both solutions could point towards a similar power drain problem
hth Andreas