This may not help and probably you have already tried. Windows Machine.
Uninstall the device in Device Manger and also the software if it allows you to.
Unplug and re-plug the UNO, maybe in a different USB port.
Just did that and my UNO shows up as :-
It did not show up in USB area.
Uninstalling has sometimes worked for me.
Having Admin on Windows would help to remove drivers and re-install.
Cannot help with MAC, no experience there.
If you have tried different computers and USB ports and different cables; maybe the board is faulty. (would not think so, though) You have said you only have one cable, I assume this works with other devices. Maybe try a different cable if you can get one.
The UNO uses a version of the ATMega chip to do the USB stuff; surface mount chip, located near the USB socket. If it is not programmed correctly the USB may not work. Have never seen or heard of this chip being programmed incorrectly.
Also, if you have access to another UNO that might help prove a faulty UNO; if it works, of course.
Cheers
Jim
