Thanks to everyone who answered, especially to those who chose option 1 or option 2. I think I know now what happened. I had a lot of minutes on my phone, about 2000 (because I had been offered some good deals on bonus minutes in the past), and when I asked my service provider the next day how many minutes were left on it, they said zero. And that was after the phone had been turned off the whole time. Every time I or someone else called my phone, it went straight to voice mail. If it had been on, it would have rung a couple of times.
The customer service people at Tracfone were absolutely clueless about how this could have happened, but the thief must have taken the SIM card out of my phone and inserted it into another phone, so they could use the minutes. That's the only real logical explanation. Unfortunately criminals are very sophisticated these days, and they know how to get away with just about anything without getting caught.
The phone itself was cheap, just $19.99. But what I had saved on it was priceless - personal messages and pictures and short videos of family, and a picture of myself with my boyfriend. Fortunately my boyfriend still has all our text messages saved on his phone, so I can copy them off that, and the picture and video texts were sent as group texts, so I think everyone in my family should have a copy. I'll have to ask them to send them again, probably to my email this time instead. And I think I can find the contacts again that I had on that phone. So I guess I can get the most important things back. It's just an icky feeling, knowing that all of that somehow ended up in the hands of someone who couldn't have cared less about it and just threw it away like garbage.