Okay... first, take a deep breath. Try to relax. It's stressful whether you have one egg or twenty, but certainly having just the one is a lot of pressure! The first thing to know is that usually chicks who make it this far do just fine whether they end up needing help or not.
The other thing to know is that the chick can live several days in a pipped egg without dying, even if it's stuck.
My duck eggs take between 36-48 hours to hatch after pipping, in general. Chicken eggs are usually a little faster, but my understanding is that 24-36 hours is still pretty normal. Usually the way it happens is that they make the initial pip, then after a few hours, they expand it some, sometimes in a circle and sometimes beginning to move around the egg as though starting to zip. Then they take a LOOOOOOOONG break and sleep, rest, absorb the rest of their yolk.
This is the hard part for we humans, because it looks like they're not doing anything, but they really are. They're storing up the energy they need to come out into the world. This is the hardest work they have ever, and probably will ever, do, so it takes time.
Now, the only thing that concerns me about your situation is that you said it began zipping. How far around the egg has it made it? If it's an inch or less, I'd say that's pretty normal for it to take a long long break. But if it's made it half way or more, then it's kind of weird, but still not cause for alarm. I've had them get stuck at that point and they usually make it out just fine eventually anyway.
The reasons NOT to help are, primarily, that the chick can bleed to death or hatch without the yolk absorbed which is dangerous for a variety of reasons. Two major things are happening during the long wait. One is that the chick has had its main nutrient supply outside its body for the past 21 days in the form of the yolk, which is attached to its belly. Inside the egg, having a mass of pulpy flesh and nutrients external to the body is no problem. Outside the egg, it is an invitation to harmful bacteria and an impediment to movement and, in a word, very dangerous. So the chick takes several hours to absorb that yolk into its body and close up the skin around it, so it is less vulnerable to bacteria.
The second thing that is happening is that the chick is shutting down most of the major blood vessels outside its body. Inside the egg, the chick obtains nutrients and moisture from the yolk and white, and delivers waste materials out of its body, all through an elaborate network of blood vessels outside its body. Many of these blood vessels are in the inner membrane that surrounds the chick. During the hatch, the vigorous work of hatching causes those blood vessels to shut down so that when the chick punches through the membrane, it doesn't also punch through active blood vessels and bleed to death.
If you help the baby too soon you will punch through those blood vessels while they are still functioning, thereby weakening and possibly killing the chick. Likewise, if it comes out of the shell before the yolk is absorbed, you expose all that tender tissue to the outside world without a protective skin, and you risk killing the baby from infection. It's kind of like having internal organs on the outside--not a good thing.
Now, I'm not saying you shouldn't help the baby. But if in doubt, it is usually safer to wait, especially since it's only been a few hours. After 24 you have my permission to BEGIN worrying.
Edited to correct typoes.
