I left my iphone by my window and now it won't turn on?

So i have a fully updated iPhone 4 with a few cracks and i sat my phone on the windowsill and i came home and it would turn on i know nobody touched it since nobody was home so was it sun damage and if so could my warranty replace it and i believe i also have my upgrade how much would it cost to use my upgrade
p.s it completely won't turn on i tried charging it and it says back

When your phone gets to hot by the tempature. Will not let you unlock your phone!
Been there done that!