Hi MartaN and Mrs N Robinson,
Thank you for posting. This issue could be happen when you press the Return button. There is no option to undo Returned students work. However, you may ask student to find the assignment to “Turn in again”. Then it will change your status from Returned to Turned in.
If anything is unclear, please feel free to let us know and we will be glad to provide further assistance.
Best regards,
Shyamal