We need more than "it's not working" to be able to suggest a solution. Did anything happen before it stopped (a fall, a software update), or has it ever worked? If you have more information, please post again as that will help us identify the cause.
Otherwise, all I can suggest is to check your settings to make sure it's not just turned off (ringer settings, notification settings, haptic feedback settings). If it vibrates for some things but not others then it's a software/settings problem. If it is enabled and isn't vibrating for anything at all then all I can suggest is taking it in for repair, as it may be that the motor has broken or become disconnected.
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