I've had that happen while removing old ones. Don't leave it in there. You'll just have to be patient with a very long skinny screwdriver poked through the coolant holes and moved around until you find it. A flashlight shining through an adjacent deck hole will help you see it. It'll probably be wedged a little and you'll have to pry it until it comes loose - then rotate the block until you can see it, lift it to the bore and then grab it with vice grips to pull it out. Anything other than vice grips and you'll be sorry, because you'll probably lose it again.

It also may be necessary to remove more freeze plugs to get at the lost one. I know you don't want to pull more, but that's what may need to be done so you can lift with the screwdriver through one hole and grab the plug through the other.

Frank