Mark, you have not told us when it happened and what you have done thus far. The very first thing you want to do is to not try to restart the phone. This may further damage your phone. The next thing to do is to clean it. Disassemble your phone using these guides. Then clean every part with +90% isopropyl alcohol. Follow this guide, and even so it was written for a 3G all the points are still pertinent to your phone. While you clean the parts carefully check for any corrosion, missing or damaged looking component. Remove all EMI shields to properly clean it. Once it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Once all this is done, reassemble your phone and reevaluate. If there is no change you could start by replacing the front assembly. Water damage is the toughest to diagnose since virtually any component may have failed. The longer it takes to clean it, the lesser the chances of reviving it.