Did it ever work? Does COM5 disappear when you disconnect the board? If not, it's not your Arduino.
What does that mean? Does it mean that you tried the Loop-Back Test Instructions?
If so and if your Uno uses the CH340 TTL-to-USB converter, that test is not conclusive. What is the TTL-to-USB converter on your board (the chip closest to the USB port)? If the device shows in Windows device manager, you can check the VID and the PID and next look it up on the web to see which manufacturer/chip you have.