Take the hint that they don't want to talk to you right now. They could be busy, or sleeping, or just not feel like talking, so just find something else to do. They'll text you back when they get a chance, and if they don't ever text you back, and it's happened more than a couple times, then odds are they don't really want to talk to you, and it's time to move on to something more productive.
Is this someone that normally would text back in any situation and you are worried about them? If it's a friend/acquaintance not replying they are either busy, didn't receive it, or don't won't to reply. Don't send anymore.
Don't text again until you hear from them. Like it has been mentioned, they might be busy, tired, or avoiding you. You can't change any of those things by continuing to text. In fact, you'll make it worse. ***If you are worried about their safety, call or go visit!