there's a crossed out ringer on a contact's text. How do I change it to not crossed out?

The crossed out bell icon means that incoming messages will sound a notification. You tap that icon if you want to turn off future sounds for that contact's texts and it will change to a bell with no cross out.

It seems a little counter-intuitive (to me), but the icon indicates that you tap it to make what the icons shows happen in the future, rather than what the current status is.

To be clear, open Messages and on the main screen in messages, swipe left on the contact in question and then tap the bell icon to change it.