water damage- black screen but phone still works

It sounds like the connectors at the point where the motherboard & LCD connect have become corroded. You will needto open the back up and examine it with a magnifying glass for corrosion spots you'll need a experienced phone person to go through it with alcohol and clean the entire inner parts, and put them into a electrolysis cleaner. If by some merciless from above there was no water damage to the digitizer connectors or all the other parts that have had 6 months to rust away i would say clean your way through it with rubbing alcohol 93% and up you might get away with the cell phone crime Of the century. It may be a single cable connector loose or that has just enough corrosion on it that wiggling and wiping might restore the voltages. A cleaning by a local phone repair shop shouldn't be but $40-$60 bucks. Cheap for what he will be able to tell you about the amount of time you will be able to use the phone in the near future if your desperate fora phone. Might just be that you go on ebay and buy a LCD &digitizer & swap it out and it will work perfect. But it's all riding on what you find when you open that puppy up. Use the walk through tutorials on here and the u-tube videos to walk your way through the process there even ones on cleaning and fixing water damage.