Can I claim my letter back from the post office? URGENT?

I don't know if I can trust what my friend told me. I left my oyster card at her house and she told me she would post it to me. She handed into the post office and it still did not get to me within 5 days. Then she told me that her friend happened to work in the post office and he checked that she wrote the wrong adress and he took the letter back to her. Is that possible or not? Do you need evidence to withclaim a letter? Can any member of the post office take anyones letter for them? This is urgent and I really need your advice! Thanks x