sleep deprived

Well. Elijah had been sleeping through the night since he was 7 weeks old. All was going so smoothly. And then last night he woke up at 12:00 and again at 3:00. What's that about? He ate fine before I put him down around 9:30. He's not sick as far as I can tell. Manny says he's sticking to the schedule perfectly - eating on time, having awake time, then nap time - just like he's supposed to. I just have no idea why he couldn't sleep last night. Naturally, I'm exhausted this morning. Is it normal for a baby to suddenly revert to his non-sleeping ways? Does anybody know what the problem could be? What can I do?