Negative base current in NPN BJT
you just reverse biased both diode(B-E &amp; B-C) . Now question about the magnitude of the current.

If you have very little amount of current like micro ampere , then you just reverse biased both diode, on the other hand emitter and collector voltages are higher then base voltage.

If current is high like mA or more, you not only biased them reverse but also reverse bias voltage is more then break down voltage. And most likely your transistor is going to burn.