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Have you powered down and pulled the battery for up to 10 minutes?

Any recent changes, new apps, updates to apps?

Running Kit Kat or Lollipop? If Lollipop how recent was the update to Lollipop?

Any known damage, drops, water, etc?

Try booting in safe mode to see if it is app or hardware:

1. With the device powered off, press and hold the Power button (located on the right edge) until the Samsung Galaxy S5 logo appears then release.
2. With the Samsung Galaxy S5 logo on the screen, press and hold the Volume Down button (located on the left edge).
3. Continue to hold the Volume Down button until Safe mode appears in the lower left of the screen then release.
   This may take up to 45 seconds.

OR

1. With the device powered on, press and hold the Power button (located on the right edge) until the Phone options prompt appears then release.
2. Select and hold Power off until the Reboot to Safe Mode prompt appears then release.
3. Tap Restart.
   The restart process may take up to 45 seconds to complete.
   Upon restart, Safe mode appears in the lower-left of the unlock/home screen.