Text message notification bell with slash through it?

If there is no slash through the icon when you swipe left (and it says "Unmute" below it) that means that the message thread is currently muted. You then tap the icon to unmute it.

In other words, the icon there shows the action that tapping it will accomplish, not the current status.

Re: "Text message notification bell with slash through it?"

Seems to indicate that you may have accidentally turned text message notifications off

Re: "This only happened to one of my text messages and I didn’t change anything. I don’t hear any notifications when I get a text now. I’ve gone in and there is no slash through it when I swipe left."

Am not sure where you looked already:

You might want to check whether your settings need changing :

eg: see if notifications are turned off, and if so, toggle them back on.

Apple Support's online article: Turn notifications on or off for a specific app - Apple Support

Eg: says:

"With iOS, you can customize notifications for specific apps.

1. In the Settings app, tap Apps.
2. Tap the app that you want to turn notifications on or off for.
3. Tap Notifications. If Notifications doesn't appear, the app might not be able to send notifications.
4. Turn Allow Notifications off or on."

There's more info. if needed in the linked article above, and in the one below:

See how to: Change notification settings on iPhone - Apple Support