Clock Doesn't always turn on

Alright, so i'm sure this is a common problem amongst many of us. But I'd like to know the cause so that I can fix it.. The digital clock turned on for the first time (since i've had the car) yesterday, then when I went out later that day, the display was off again What's usually the cause of this? I already checked the battery, so that's not the problem. (12.5-12.6V)