Relax! Tell a station attendant with a detailed description as soon as possible. They can contact the train driver who will check for for your item and it is highly likely you will get it back. I've left my passport and cellphone (with ¥10,000 in the case) on trains on separate occasions and both were returned to me in perfect condition! Japan is a very honest country :) You may have to ride to the station where they are keeping your item in order to claim it, but that wasn't a big deal for me~