指定席のチケット、なくしちゃった。 — I lost my reserved seat ticket. (Transit · Shinkansen, Casual — to friend, JLPT N4)

Gesture & etiquette

Looking concerned while checking your own bag, then patting your friend on the shoulder to reassure them.

Do not panic or cause a scene in the train carriage. Avoid being rude to the train staff when explaining the situation.