There are a few things that can cause your Brushless DC Motor to stop running or run incorrectly. The Hall sensors of the motor can be bad or shorted. One technique that can be used to check if the Hall sensors are bad is to pull up each Hall with a resistor to 5 volts, check each Hall with an oscilloscope while spinning the shaft.
You will need to repeat this for each Hall, meaning you will have to do them individually. A low and high signal should appear on the scope as you spin the shaft manually. Keep in mind, that it does matter what value is used for the resistance, it all depends on the amount of current your Halls can take. If you find that your Hall Sensors are working correctly, you can move onto the next step and check the Phases of the Brushless DC Motor. Hook up the motor to a Driver/Controller. With an oscilloscope, check each Phase to see if a switching signal is present. If you find that the Phases are also doing what they are suppose to be, move on. The bearings of the motor could be a possible problem along with internal shorts of the motor and if the motor is still not working properly, you might want to consider getting a new motor.