I have same trouble -
all works fine. I have Win10 home. From one a day after start Arduino IDE as usual, COM 4 is missing. And I am not able upload new program.
I am using Arduino DUE.
I searched on whole internet, but no idea, what I must to do. There is no help. How to install COM drivers. Drivers it self are missing to me. No idea how to download them.
Please can anybody write some FAQ about this? Will be realy nice.