Why won't my iPhone turn on after one night?

Yesterday at around 6pm, I went to bed. Normally I plug my phone in before I sleep, but I forgot to this time. It had at least a 30% battery. When I woke up an hour ago, I tried to open my phone to check the time, but no button responses worked, and the screen was not turning on. I have held the power button for 20 seconds, and held the power and home for 20 seconds. I then plugged it in to charge immediately, and it is still plugged in. Why won’t it turn on after only a few hours? What could have caused this?