This is quite inconvenient, suddenly there is no notification sound and lights for any events, like hangout message, GMail. Even the notification is displayed on the status bar.
First thing to check is if you have Quiet hours (Under Sound setting) enabled and current time is between the start and end of the quiet hour.
If current time is not quiet hours and even you disable Enforce quiet hours, there is still no notification sound and light. Next thing to check is if you have the QUIET HOURS tile turned on in the Quick Setting Panel.
So to solve this problem for now, you just need to tap on the QUIET HOURS tile to disable it (grey out). This should fix the problem.