Look in your sound & notification settings (or whatever Samsung call it, as they rearrange the Settings in their own way). Lollipop includes the option to schedule a silent or priority notifications only period, so the first thing to check is that you haven't got that set.
There are other apps that can do this (any automation app, such as Tasker, Llama, Macrodroid, as well as various third party profile apps), but I'm assuming that you'd know if you'd installed one of those and set it this way, so the system settings is the most obvious place.