Smart Boards may be locked up as a safety measure due to incorrect operation. There could be many reasons for this, such as riding for extra long time, rider weight exceeds max limit, riding over speed for longer duration etc. Please refer to instruction manual for details.
In your case you should try soft reset to factory default by following instructions;
1. Long press the power on button, until you hear second "beep" or "sound'. (You will notice motor light and front lights flashing)
2. Wait 5 seconds, and then power off the scooter, and then power on again. The balance system should be reset now.