I left my souviner bag full of seaweed

I left my whole bag of seaweed i bought from korea on the narita express nex going to tokyo what should i do?

TEL 050-2016-1601
（8：00〜20：00）

There also is an official English phone line for JR.

(050) 2016-1603
or
+81 (50) 2016-1603 if you're calling from a non-Japan phone line.
(Same thing with the other line that Ken provided. You need to add +81 and delete the first 0 when calling from overseas phone lines.)

Hours of Operation for JR East Infoline
10:00 to 18:00, every day of the year except during the year-end / new year holidays

https://www.jreast.co.jp/multi/customer_support/infoline.html