First, make sure the phone is turned off immediately after retrieving it from the water, and leave it completely powered off for at least 24 hours, until it is COMPLETELY dried out otherwise you will fry the delicate electronic components inside it.
Rice will remove most of the external water, but some may remain internally.
Remove as much of the covers, battery, etc as you can, then leave the phone in a warm, dry place (e.g. an airing cupboard) for another 12-24 hours.
Then try turning it on again. Let me know how you get on...
It could have been caused by an electrical short due to the water, or any number of reasons.
Hi, just checking in on your question. How did things turn out for you?