Hi James_H.
Effectively James_H., as the Galileo Getting Started and https://communities.intel.com/servlet/JiveServlet/previewBody/21838-102-6-25157/Galileo_GettingStarted_329685_004.pdf guides state, connecting the board through USB cable before powering it may damage your board. You must always connect the board to power before connecting it via USB to your computer. Depending on how much time you left it connected may permanently damage the board but there are options to try if you left your board connected only via USB. You can try to update the firmware of the board that may make the board work again. The procedure for updating the firmware is described here https://communities.intel.com/servlet/JiveServlet/previewBody/21838-102-6-25157/Galileo_GettingStarted_329685_004.pdf , Let me know if this works.
Regards
Sergio