As previously mentioned, definitely book the watch in to be looked at by your nearest Genius Bar. They will conduct an evaluation of the watch and give you your options. If the watch is deemed to have an in warranty hardware issue or an issue covered by one of apples repair programs then you may be able to get it serviced (potentially replaced) for free. If your watch does not meet these criteria or has any signs of accidental damage then you may have to pay for this service. The Genius Bar will be able to give you a quote for how much this costs, the cost will vary depending on the series of watch, the size of watch, the materials (ie. aluminium or steel) and whether the watch is a cellular or GPS only model. As always with a Genius Bar reservation, make sure you know your Apple ID and password before arriving to ensure a good swift appointment.