In order to reset or reccaliberate your hoverboard, you need to make sure your hoverboard is turned off and it's sitting level with the ground on its wheels.
Without moving your hoverboard, press and hold the power button for 10 seconds. Ignore all flashing lights and beeps in this 10 seconds. Then let go of the power button and turn the hoverboard off. The hoverboard is now reset and ready to be turned on.
If you have the DUO Series hoverboard like the one in the image blow, you would need to turn the hoverboard upside down and level with the ground before holding the power button for 10 seconds.