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.