It's not a common problem. Never has been a common problem. It's a problem for people who have problems, and there will always be people who do have problems. Some of the reasons are rare, some not-so-rare - battery needs to be charged, exposure to liquids, in simple need of a hard reset... relatively common things that can affect any model of smartphone or tablet.
Yeah, it's under warranty. Call or chat Apple for assistance. Get an appointment at a store.