Are they bantams or a smaller breed by any chance? They usually develop more quickly than the larger breeds. If not, then such an early pip would most likely indicate that your temps have been high, which usually speeds up development. Either that or this egg had started to develop already when you first put it in the incubator. That seems to have been happening quite a lot recently with the hot weather some parts of the US has been having.
I'd say leave it alone and see how it does. There's a small chance that because it had already pipped when you had the incubator open, it might have shrinkwrapped and be stuck inside the shell. But there's also a much bigger chance that it could go on and hatch normally all by itself, and by opening the bator to help this one before you even know if it's actually necessary, you would be putting all the other ones at risk...
I know how hard it is, but leave it for a while and see what happens.