Let it try out (leave in a warm spot for a few days). If that fails, you have to have somebody open it or try it yourself
iPhone Repair Guide has instructions on how to open it. Try to find any remaining corrosion inside and remove it with a brush and some alcohol or hydrogen peroxide solution. Let it dry again in open condition. Measure the battery to make sure the battery fuse didn't kick in. If the battery voltage is 0, use 5V charger and connect it directly to the battery for a few seconds to reset the electronic fuse. You can do so by using and old cell phone charger, cutting and exposing the wire, then connecting it to the battery pins (watching the correct polarity - red to plus - black to minus). Feel free to post again if you need more details.