I can send but cannot receive an sms on my iPhone 5s-this just came about out of the blue!
Apple doesn't have a great article on troubleshooting SMS reception so your first course of action would be to contact your carrier or look on their website on how they wish users to troubleshoot SMS reception.
Once you've done that, you could go over this article:
It covers messages, so you might want to disable iMessage entirely on that phone while you work out what's wrong on the carrier side or setting side.
Feel free to ask a second question once you have some troubleshooting done and be sure to include your iOS version and what resources you're using to go through the checklist of things to remedy this.