Com Port error

I have 2 UNOs. I can upload a program to 1 of them, no problem and it uses COM4. When I try to upload the same program to the second UNO it fails because it can not fine COM3 port. However, the port selection is grayed out and I can not select a COM3 port. The second one worked ok before using COM3 put now COM3 is gone. I don't know why 2 UNOs are using 2 different ports. I am using Windows 8.1 and can not find out how to see the port on the system. I am new to Windows 8.1

Bill

It seems your computer is having problems communicating with the defective board.

Check to see if you can get the device into DFU mode. If you can, have a go at reloading the usb-serial firmware through Atmel's Flip.