If you use a USB (as opposed to a RS-232) cable, then note that the COM port number that the software looks at needs to be the same as the COM port number allocated by the PC to the USB/serial driver.  Suggest you check on "device manager" what the COM port number is and change your app software if it is different.  A PC can allocate a different COM port number each time it is switched on or when something is changed.  Good hunting...