I wonder if anyone has some advice on this problem. One of the reefing lines was pulled back inside of the boom; the blocking knot at the end came undone and before I knew it the end of the line was inside the boom.
I had luck using a electricans fish, on a mast with internal lines. The fish is a thin metal ridged wire on a reel. You can get one at hardware store. Run it through the boom and tape the end of the fish to the reefing line and pull it back.
In the past I have used small diameter pvc pipe and fed it in from the aft end of the boom. I tie a pigtail through a hole drilled in the end of the pipe and which I fish out with a hanger. It is very cheap and stiff and flexible enough to work well. The one caviat is that you need a boom end fitting with and access area.
Tanks for all the suggestions. I have an electrician fish tape so I will try that; I also think I may try the small diameter pvc technique. The line that is inside of the boom is the luff reefing line which I think it is attached to a plate that moves inside.