We understand that trying to build the cable something went wrong with the voltages, in this case since the power supply connectors are built into the board this shortcut may affect the board itself. What you can try as the last course of action to make sure the board is not working properly is set the bios defaults with the jumper J6D1 and also trying with a different power supply besides the ones you are using.
You will find the jumper location on page 92 of the following document:
Other than that if still doesn't work the board it's already damaged.
Hopefully it will be just the board, but let us know how it goes after trying the steps suggested or if you need further assistance.