HTC One M8 User Manual page 244

244 Sync, backup, and reset
HTC One not responding?
If HTC One does not respond when you touch the screen or press
the buttons, you can still restart it.
Press and hold the POWER and VOLUME UP buttons for at least 10
seconds. HTC One then restarts.
Resetting HTC One (Hard reset)
If HTC One has a persistent problem that cannot be solved, you can
perform a factory reset (also called a hard reset or master reset). A factory
reset reverts the phone back to its initial state — the state before you
turned on the phone for the first time.
The following will be removed from the phone when you do a factory
reset:
Your Google Account
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All accounts you've added in Settings > Accounts & sync and the
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data associated with these accounts
Personal data that you've created or synced to the phone such as
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your profile, contacts, messages, email, calendar events, and more
System and application data and settings
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Downloaded apps
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Files such as your music, photos, videos, and documents can also be
deleted from the storage, if you chose to do so.