Hello. I have the same problem as whom sent they problem and I examined all the solutions in all threads with problems like mine.
and what the problem is ....
I was working with my arduino UNO R3 original and then I figured out that it's not working anymore and IDE can't upload the code. so I plugged the board out and again plugged in. then there was no sound from windows because of plugging the board in. then I went to the device manager and I saw that there is no COM port recognized by my PC. and the status of my arduino in this situation is that the green and orange light labeled as 'L' are constantly on while it's connected to either USB port or adapter.
as I said I examined another cable, another PC and lots of other solutions in the forum but none was worked!
If you have tried other cables known to work and other USB ports and windows still does not see the board there is a good chance you did something to damage it.
Do you have access to another computer to try ?
Generally caused when you apply incorrect voltages to the wrong pins or abuse the power rails.
The USB to TTL serial adapter on the board is what causes the COM port to be recognized. On an original Uno, that is the square black chip marked MEGA16U2 that's right behind the USB jack. Due to the IC package, it's a very difficult job to replace the chip. You also have no guarantee that there isn't damage to other components on the board. Usually it would be best to just replace the board if the ATmega16U2 is bad.