To Perform a Master Reset via Hardware in HTC One A9:
If you cannot turn the device on to access any settings, you can perform a factory reset by using the hardware buttons on the device.- Ensure the device is turned off.
 - Press and hold the VOLUME DOWN and the POWER button together.
 - Wait for the HTC download mode screen to appear, and then release the POWER and VOLUME DOWN buttons.
 - Press the VOLUME DOWN button to select factory reset.
 - Press the POWER button.
 - The device will now reset.
 
