Mobile device doesn't vibrate (Samsung)

If the mute function is enabled in the Motions and Gestures menu, all the incoming calls will be muted if you cover the phone's front part (the screen) with your hand or with any flat surface. To prevent this from happening in future, disable the mute option in the Motions and Gestures menu.

1. Go to Settings and tap on Motions and Gestures.
   
   

   
   
   

2. Move the slider next to Mute into OFF position.
   
   

   

Maybe you have accidentally turned off your mobile device. If this is the case, all you have to do is to turn on your mobile device.

Maybe your device doesn't ring because the blocking mode (a.k.a Interruptions) is enabled.

In order to disable this mode:

1. Go to Settings -> Tap My device.
2. Scroll down and tap Blocking mode.
3. Move the slider into OFF position.
   
   

Note: On Android 5, Interruptions is the feature that replaced the Blocking mode. In order to disable this feature:

1. Go to Settings -> Select Sounds and notifications.
2. Tap on Interruptions -> tap On receiving calls and notifications.
3. Select Always interrupt.

If your device doesn't vibrate at all and its battery is removable try restarting it by removing its battery and reinserting it after 2 minutes.