What can people do when they don't receive a response to a message?

Well, I think the most practical thing is to send a polite follow-up message after a reasonable wait. People get busy, especially at work, so emails or messages can easily get buried. For example, in my job as a UI designer, I often have to follow up with clients when they don't respond to my design drafts. I'll wait about three or four days, then send a short message just checking if they saw my previous email. Usually they appreciate the reminder rather than finding it pushy. That said, if there's still no reply after two follow-ups, it's probably better to move on or try another contact method.