Don't panic, stay calm.
Observe whether the self-service book returning machine works normally. If the machine breaks down or is abnormal, you can immediately ask the staff of the self-service lending and returning desk in the library, or get in touch with the customer service staff of the online library in official website to explain your situation.
If the self-service book return machine works normally, it may be that the label of the book is damaged, incomplete or improperly placed, which makes the machine unrecognizable. At this point, you can check whether the returned book label is intact again and try to reposition the book to ensure that the label can be correctly recognized by the machine.
If the above methods can't solve the problem, you can contact the library staff or administrators to explain the situation to them and ask for help. The staff will give solutions, such as emptying the borrowing basket in the self-service book returning machine, confirming the book status, and finally registering the return.
In a word, readers should keep calm and don't panic when the self-service book returning machine swallows books. This problem can be solved by checking whether the self-service book returning machine works normally, re-checking and placing book labels, and contacting library staff or administrators.