What is my phone doing? And how do I fix it?

Ok so my phone does this thing sometimes for maybe about 20 minutes tops every so often and it started after I dropped it in the pool about 6 months ago, well it actually didn't start doing it again until a month ago and it's really annoying but I've learnt to deal with it. But today it won't stop. Ok sorry I'll get to what it's doing.. Ok so out of no where it will power down and the screen will go black and it won't let me turn it back on, but when I plug it in like as to charge it, it shows the little battery icon which is telling me my phone is dead when it was charging all night, and then after about 7-10 seconds it will say it is powering back on by showing the white apple in the middle of the screen, that lasts about 10 seconds as well and then the process starts all over again. And usually after a little while it will just turn my phone back on but this morning I was texting my friend when it did it again so I plugged it in and it started doing it and I waited a few hours and it was still doing it, so I left it at home today while I went out and when I got back, it was still doing it and is as I am writing this. It has been doing this now for about 9 hours and I need it to turn on and it is really bugging me. The second I unplug the phone it completely turns black and when I plug it in it continues acting that way. I've tried everything I can't think of, I've tried holding both power buttons while it is charged and when it is not and it doesn't seem to do anything. Please help if you know what is happening to my iPhone 5c.