It's the weirdest thing. The phone was fine and I wasn't installing any new software. I was using a notoriously buggy app (Words With Friends) and it was crashing a lot, so I decided to reboot the phone because sometimes that seems to help. The phone shut itself down, but did not turn on afterward. It would not turn on when I held the power button. I figured it was out of batteries, but I just connected it to the charger and that's not the problem (not even an indicator light when I try to charge). So I opened the thing up and made sure the battery and the microSD card were in the right places and there wasn't any dust, but to no avail. To clarify, absolutely nothing happens when I try to power it on, the power button is working, and this is not the result of installing any software.
So, what is wrong? Thanks so much for the help, everyone.