What I think is that you have left it for a while?? Thats what happened to me first it was fine, then I left it at home while i was thinking of a name (it is now ron but he died *sob*) I came back and he was crying! I hope this Helps!
The night before, the parent left. Now you need to name the baby, and you'll raise him just like you did with his mother/father. He is crying because, well, wouldn't you cry if you awoke to find that your parent/guardian left you? Raise him well, and when he's old enough, he'll have baby(s) too! Congratulations on your next generation of tamagotchi