If it has gotten wet check if the [https://www.verizon.com/support/knowledge-base-301128/|LDI] (liquid damage indicator) shows that water has penetrated the phone.
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''If it has'' then the phone will need to be opened and checked for any corrosion has occurred which may have damaged the components.
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If it hasn't gotten wet or been dropped here's the [document|41458|troubleshooting guide] from the service manual.
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Go to p.20 (of 75 pdf) ''6-3. Power, Charging'' and follow the steps shown to see if you can find out what's wrong.