you should of heard or felt something if it knocked the teeth off the gear, it could also be a broken shifter fork. you don't really know for sure unless you tear it down. If it was my bike I wouldn't ride it. If you do drain the oil, strain it in something to see what comes out. I dont think the flow of the oil draing will be strong enough to wash out the medal teeth if that's your problem. you can also use a flexable magnet to try and fish some of the medal out to get an ideal on what is broke.