Switching Piezo Buzzer with a Transistor
Make sure that the GND connection at the emitter of the transistor also connects back to the GND of the Arduino board. 
Check that you have the diode installed the right direction. If the diode has a line or band marked end that side should be connected to the +5V.
Normally you would check the voltage when the buzzer is in place instead of removing it. Since you removed the buzzer and measured less than a volt it is an indication you have the diode installed backwards.
If you had the diode installed backwards replace your transistor with a new one. You likely damaged it.