I have seen this issue several times. If restarting your PC and the Galileo does not work.
Unplug your Galileo and follow the instructions in Overview | How to Find Hidden COM Ports | Adafruit Learning System
Uninstall the com port assigned to you Galileo then plug it back in so that a new COM port is assigned to it.
You can also usually sometimes get away with simply, plugging the Galileo to a different USB port on PC because it re-enumerates and assigns a new COM port for your Galileo