why did my iphone 3gs go black and now cant get it to turn back on? please help!!

my iphone 3gs was fully charged i went to my girlfriends house in the morning and it was working fine all day around 3 oclock it suddenly just off and went black i tried the "cold boot" that didnt work i tried charging it in the wall and with my computer i tried to restore my phone but itunes wont recognize it ive also tried holding the home and power button simultaneously waiting 10 sec while it is plugged into the computer releasing the home button and waiting foritunes to ask me to restore my iphone but even that doesnt work please someone help me

ps the iphone is only about 5-6 months old