My Torch ran out of battery completely, and when I recharged it and turned it back on, all my e-mails and text messages were gone.

Any idea how this happened, and how to prevent it happening in the future?

It shouldn't have happened.

But 1) don't let your battery run down below 20% and 2) backup your data frequently.

As M-M says, I don't think the battery running dead would delete the messages. Had to be something else.