The buttons on my Iphone 5 are now unresponsive, all of them. The touch screen still works fine but the buttons are dead. Any ideas?

Today I was charging my Iphone 5 and i noticed that out of nowhere all of the buttons had now became unresponsive. Nothing will work so i cant even get into my phone unless i plug it into a charger, and even then im stuck in the settings app because i cant use the home button to get out of it. This phone has never been dropped or beaten in anyway it has been in an otter box defender series case since the day i bought it. So i am sort of confused as to why this would happen or how it would happen. Has anyone else ever experienced this before? And if so what was your solution? Any ideas? Thanks.