for those who can't fix it with a reboot:
also. there are three (or four) prongs in the jack, one at the top right (when you look down with screen facing you). I was able to fix my daughter's headphone jack by carefully pulling the top prong outwards a little to give better contact (using a needle). Other prongs are on the left side. You may be able to do the same with the second one from top. But chances are small that this will cure your particular issue.
When you can hear sound only from one side, try to pull out the headphone cable one notch. On my daughter's phone you would hear on both channels now, however mono. That would indicate that the prong on the top right is pushed in too far and doesn't make good enough contact.