Iphone 4 fix, not working Help!

I have a cracked screen on my Iphone 4 right, although it has been working fine for the last 2 months… I dropped it the other night, and it just automatically turned off, i plug it into the charger and it doesn't charge, I try put into restore mode it doesn't do nothing. I plug it into computer and it says unable to install device… Any help?

Added (1). I dont think it is under warranty…

Is it under warranty?