It looks like either the Serial bridge or the PC USB port have blown.
Have you tried powering down PC then starting it again.
Other things to try:-
- Plug something else in the USB port to see if it is working.
- Plug the arduino into another computer.
- Change or test the USB lead.
- Use another USB connection into the PC, many PCs have more than one free. Try swapping the mouse with the suspect port.
However it does look like you have blown up the PC especially if you reverse powered it.