Problem with self balancing robot- wheels only spine one direction

Hello everyone,

I have finished building the circuit for my self balancing robot, yet it is not working as intended. After connecting the power source, the dc motor wheels spin at a constant speed and only in one direction, regardless of how the MPU6050 is tilted. My serial monitor has also been reading weird letters and symbols when I have the circuit connected. I believe I have added the PID libraries correctly to the Arduino library, as the sketch successfully downloaded to my arduino, but I am not 100% sure. Any help that can be offered would be greatly appreciated. I have posted a link where the code project details, and the circuit diagram can be seen. DIY Self Balancing Robot using Arduino