Some time Hard reset is needed to reset mobile to factory default settings in case of pattern lock forgotten or slows peed of operating system. If we forgot pattern lock and enter wrong pattern many times then mobile shows on screen enter your gmail account to reset pattern lock. Then if you have remember your gmail account then you can reset easily and your data wil be safe being erased. If you not remember your gmail account then we can reset mobile by key combination. This is easy method just follow the steps below.
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.