My two guesses are dead battery (possibly finished off by being off for several months) or fried "other component" due to water damage. While the water may not have caused an immediate short, the water may have caused corrosion which built up in the time the phone was inactive. My suggestion would be to open the phone and due a visual inspection to see if anything is noticeably damaged. If all looks ok, try replacing the battery first. Let us know what you find.