no big deal
It happens to a lot of us who forget now and then to remove the ring that holds the extractor on. Look back into the reciever and use a small screwdriver or a punch to wiggle the ring past the bolt stop and ejector. Pull on it slightly and it should come out without too much fuss. Next time you remove the extractor, remember to not put the bolt in without the extractor ring removed. It will easily bend to remove and bends back without too much trouble.
One side note, most of the stocks used an oil finish. Just sanding the stock down to bare wood doesn't completely remove all the oil. Depending on what type of finish you decide to use, you may need to remove all of the oil. I use heat to make the oil back out and wipe it off with a wag soaked in degreaser. A heat gun or a wall heater will do fine. It will take several heatings before all of the oil is removed. Some folks use an oven cleaner, but I hate the smell myself. I use Arrow's wood finish myself. You don't have to worry about the previous finish with it. Just rub it on a little and then sand it into the stock. A few coats and your done. Good luck with the gun.