The tricky thing about the boards with native USB functionality like your board is that the USB code that creates the CDC serial port is running on the same microcontroller as your sketch. This means your sketch code can break the USB code, or stop it from running. When that happens, it no longer presents a port.
That makes it so you can't upload normally any more. However, the situation is really not so bad because there is an independent program called the bootloader in a separate section of memory from your sketch, and that program has its own USB CDC code. So even if the sketch is completely broken, you only need to activate the bootloader and you will get a port back and be able to upload.
Now, the question is how to activate the bootloader. There are two methods, but the easiest one is not available on all boards, unfortunately I'm not familiar with the itsybitsy, so I can't say whether the easier one is available. It's worth a try though:
Press and release the reset button on your board quickly twice. You should now see the LED on the board pulsing, which means the bootloader is running. The double press causes the bootloader to run indefinitely (until the board is reset, powered off, or an upload is done), which means you don't need to get the timing of the reset just right.
Select the port of your board from the Tools > Port menu. The port number may be different when the bootloader is running so don't assume you already have the correct port selected.
Start an upload in the Arduino IDE.
The upload should now finish successfully. After this, you should be able to go back to doing normal uploads without needing to press the reset button. If you still need to do the reset trick to do uploads after this, the problem may be caused by your code. You can verify this by uploading a simple sketch like File > Examples > 01.Basics > BareMinimum.
If you don't get a pulsing LED or a port after doing the double reset trick, let me know and I'll provide you with the instructions for the less easy (but still super easy) other technique.
A quick double tap does make the red LED pulse. And when it does, the com port selection in Arduino IDE becomes active and shows Com Port 5. But when I attempt to upload the bare minimum sketch, it fails and generates the following error.
avrdude: ser_open(): can't open device "\.\COM5": The system cannot find the file specified.
Problem uploading to board.
I double pressed the reset button like suggested. Then I began an upload of the bare minimum sketch. And I double pressed the reset button again. This time the upload completed.
And I'm now able to upload other sketches as well.
Even though it is fairly easy to break the USB stack on the native USB boards (for example, if your sketch puts the board to sleep), once you know the technique it's simple to recover from this situation so it's really not such a problem. Of course, then you have the fun process of trying to find what it is in your sketch that is the culprit.