She realized to her panic that one of the tickets was missing.
Did someone with her accidentally take it?
No, she was the one and only traveller.
It is understandably frustrating to lose a ticket. She should contact the agency that she bought the ticket from for support. They may be able to replace it.
I don't understand why she is so upset, it is just a ticket.
Imagine that you lose a ticket to a show you really wanted to watch. You would be upset and panicking at losing it as well. Hopefully she can contact the seller and get another one.