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Switch off your phone and disconnect any charger, open all flaps and let the phone stand for 24-48 hours in a well-ventilated area, or in a bag full of rice. This may help to get rid of any moisture left inside the phone. Try to start the phone and see if you still face the same problem.
If the problem persists you may need to contact your Local support and get your phone inspected by a professional, but bear in mind that water damage is not covered by warranty at most distributors.