Why won't it turn on?

Never put a device in rice the starch in the rice causes havoc in the phone . sounds like the corrosion has set in on the circuit board and is shorting out the phone You may be able to save the phone by cleaning it thoroughly

Clean every connection with a soft toothbrush. As well clean the logic board; use a 90+% isopropyl alcohol for all cleaning. Now is the time to start drying out the phone. Set the screen in a desiccant or silica gel leave it in for several days moving it from time to time to get the max effect from the desiccant. Use a blow dryer on a low heat setting to dry the rest of the phone. Only use a hair dryer once you have cleaned the phone completely with the alcohol. Do not attempt to dry the phone if it’s still wet with water with the blow dryer as this only spreads the water. Replace the battery with a new one considers the old ones done. Clean again with isopropyl alcohol let dry and when you think everything is clean and dry reassemble the phone (make sure to clean the contacts on the screen after removing from the desiccant). Once you've reassembled the phone it’s time to power it on. Hold your breath and hope for the best. you may also find this guide helpful

Electronics Water Damage