Sometimes, Amazon can flip shipments to "Receiving" or "Shipped" status before you've actually finished them in Seller Central. We typically see this happen more during peak season.
The good news is, once the shipments are flipped back to "Working" status, they can be brought back into Stratify in order to complete the box-level content information and send it to Amazon.
Since this is an Amazon issue, what you'll need to do first is contact Seller Support and make sure you're speaking to an FBA Specialist. Let them know your shipment flipped to "Receiving" or "Shipped" but you haven't actually finished or shipped the shipment yet, then request that it be switched back to "Working" status.
Once they make the change, navigate to the List>List & Prep page in Stratify and click the Sync Shipments button next to Working Shipments to pull it back.
At this point, the shipment should appear, and you can click the 3 horizontal lines next to the shipment and select Work on Shipment..
From here, you can finish up the box contents if needed before completing the shipment.
For detailed steps to help finish up the box content, please see- Editing Box Level Content Information