I've determined that it has to do with my configuration, in some way. I have two wordpress sites, both through 000webhost. Both have been functioning normally.
The first one (and the one with the issue), seems to have had WP Mail SMTP installed, however I don't remember installing it. I also found that 000webhost does not allow SMTP.
The second one is working normally, so I don't believe it's an issue with my host allowing or not allowing emails. I even installed WP Mail SMTP, and it continued to work. So -something- on the first website has changed, and thus broken the php mail function in... some way.