I Had a Bad Day

Yesterday was bad. I was exhausted but I don't know why. I stayed in bed almost the entire time and slept on and off. Nothing helped me gain any energy. I'm not sick, and I had gotten enough sleep the night before.
When I woke up this morning, it was the same thing. I'm exhausted. It doesn't matter how much sleep I get or what I eat or drink that normally boosts my energy. I hate feeling tired and drained. I'm back at work today and I just can't function.
I want to be able to function. It's pissing me off.