mine has done exactly the same. It had no water exposure, and then wouldnt charge due to the caution sign and water drop. The phone is now flat and when I put it on charge it comes up with a circle with a lightening bolt inisde. Still will not charge. Tried a hairdryer just as an additional method byt still no luck.
First, let's check for water damage. Pop the SIM card tray out and then look at the little sticker that can be found inside the SIM card slot. If the sticker is red, pink or purple, then it was water-damaged. If, however, it stayed white, then there’s no water damage. However, there might be just some liquid residue in the port area. Try using a Q-tip to clean it up or a hair dryer to blow warm air into it. After that, reboot your phone.
I had the same problem and googled for an answer. This one below helped me:
"Usually this error occurs when you are charging your phone while the phone is off. As simple as this may sound, but simply turn on your phone first (or reboot your phone) and then plug in the charger. It’s simple as that. The moisture detected error typically show up when you try to charge the phone while the phone is off.
If your phone is out of battery and you cannot turn on the phone first prior to charging, then try holding the power button and volume down at the same time, and while holding both buttons, plug in the charger, you should see phone load up with the last bit of juice."