I know this has been marked as solved already however no solution was really posted, and hence I just ran into this issue today here’s how I solved it.
first how I screwed it up, I accidentally touched ground with the RST pin, at this moment the core rebooted and went into flashing green mode, I had it connected to my PC so after not getting any response from the factory reset I decided to try the serial port.
To my surprise I had a new unidentified device in the device manager, and part of the name was DFU, at this point I figured the core is probably stuck in DFU mode so I flashed it with the latest firmware and voila, back to business.
Hopefully this will help anyone else that may encounter the same issue.
Here’s a link to help you install the DFU drivers and util by kennethlimpc
And here’s a link to the tutorial on how to flash the core (including the download link)