If part of it fell in the diff you must have pushed it in on purpose
Do not panic it is a real simple fix, the centre bit inside of the broken bolt is a magnet get a centre punch and slighty tap it in just enough to get a flat tip screwdriver in behind it and unscrew the broken bolt it is that easy -- just push the centre piece in enough to get the tip of the screw driver in behind it.
You can order a new plug from bike bandit I ordered two the last time just in case I do it again.
You can go to any hardware store and get a temp plug in the meantime. The torque setting for the plug is if I remember correctly is 10 or 11 lbs it is not much use a torque wrench or you will break it again.