If you verify a task in Manual Review and accidentally close the tab or a browser with the unfinished task, you will not be able to get the task after re-opening Manual Review.
The reason for this behavior is the following: if some operator works with some Manual Review task, no other operator can work with it in parallel. When a browser tab with the verification task in progress is closed, Vantage still considers that the user is working with the task that is assigned to him/her. It will withdraw the task from the user in 60 minutes automatically. So, the task does not disappear completely. It will come back to the queue after 60 minutes.
Wait for 60 minutes for the task to get back to the queue.