Sometimes Leonardo Serial port will be locked by the sketch. It will lost its COM Port in Device Manager.

Inadvertently I found a method to fix this issue. There are two steps:

Step 1: Checking Bootloader

Please plug Leonardo to your PC USB cable. Open the device manager.  Press RESET button on the board.
Then Device manager should appear a COM port named "Arduino Leonardo bootloader (COM x)". After a few seconds, it disappear again. Don't worry, that is no problem. it means the bootloader is fine. If you get this information, you could go Step 2.

Step2: Uploading

Open a "Blink" sketch on Arduino IDE, select "Leonardo". Now there is no COM port. Press "RESET", ans select the COM port in arduino immediately. Wait a minute, Com port disappear again.
Click "Upload". After IDE has completed verifying, press "RESET" at once. If everything is OK, the COM port has been fixed after uploading.

Maybe you need to try several times, because the correct time to press "RESET" is not easy to be caught.
Kevin, our client, would like share a better way:

I finally found a reliable way to upload a sketch to this board. Here are the steps.

1. Plug in the micro-USB to the board and your computer. Make sure the right COM port and Arduino Leonardo are selected in Arduino application
2. Press and hold the Reset button on the board. Don't release it yet. Then hit the Upload button in the app
3. Only release the Reset button after you see the message "Uploading..." appearing in the bottom left corner of the app
4. Wait until it says "Done uploading" and then unplug the cable

Hope you'll find it useful and pass on the info to others who encounter the same issue.

See more explanation on why this works: https://www.arduino.cc/en/Guide/ArduinoLeonardoMicro

This method is more reliable.