Check the Power Button: Do you feel a slight “click” when you press the power button? If not, there may be something stuck in there and the unit will need to be serviced.
Make sure that you are not standing on the foot plate or applying pressure of any type on the foot plate. If the foot plate detects pressure, it may prevent shutdown.
If you are still having problems, remove the battery. Look for any signs of burning or damage at the connection points. If there is, the unit must be inspected by a technician. If everything looks OK, you can reinstall the battery and charge it and try to power-on / power-off.