My iphone 3gs wont turn on?

okay so i have had my iphone since the beginning of august and have had the otter box on it ever since the day i got it. i have dropped it a few times, but have never cracked, scratched, or had issues with it. today while on the bus i was playing a game. i decided to quit, so i went back to the home screen and pressed the lock button. when i remembered to text someone, i tried unlocking it and nothing would work. I pressed it several times, held down both buttons at once, waited about 30 minutes. then i came home and tried calling it, (In the process of hitting the buttons i could have turned it off,) but it rang and then went to voicemail didnt go striaght to voicebmail. I then tried plugging it into the charger for about 20 minutes and nothing is working. PLEASE HELP ME!