It will remove all the data ,contacts and apps that you installed
Note! All data on mobile memory will be lost like messages, contacts, call history. So take backup important data before hard reset your Htc One M8 cellphone.
Make sure the device is turned off.
Press and hold the volume down and the power button together.
Wait for the screen with the three Android images to appear, and then release the power and volume down buttons.
Press volume down to select factory reset as shown in photo.
Press the power button.
The device will now reset.
Htc One M8 Restore Factory Hard Reset Format Phone.So lets start the Htc One M8 Restore Factory, Htc One M8 Hard Reset.Turn Off the mobile phone for few mints.Htc One M8 Remove Pattern Lock.
Restore Hard reset will set your phone in origional factorry settings. It will remove all the data and apps that you installed. Your contacts will be removed and you have to add them again.