IPhone 4 shut off and won't turn back on?

I left my iPhone in the car (mind you, the battery was low) when I went back to retrieve my phone like 30 mins later it was turned off. I assumed that it was dead, and it just needed a charge, I was wrong (or maybe I didn't charge it long enough) but then I held down all of the possible buttons, and I still got nothing.

Try doing a soft reset by holding down the power button and the home key at the same time for approximately 20-30 seconds…

If that doesn't work, try doing that while the phone is on the charger

Thanks so much!