We have had some reports of submissions to Blackboard being 0 bytes (empty) when accessed for marking.  In the past this has been identified as a problem with a browser, initially with Edge and then with Safari.  These errors have since been fixed in Blackboard upgrades, however if you have students and markers still reporting similar problems it may be prudent to offer your students some advice for when they upload a submission:

Start the submission at least an hour before the due time.

Make sure you are on a stable, secure connection, an intermittent wi-fi connection could cause problems.

Close the document before trying to upload it to Blackboard.

Wait to see the document rendered properly on screen before assuming that it has uploaded.

If you are having problems, change browser and/or refresh the cache in the browser you are using.

Check that your internet connection is still active and you are still logged in to Blackboard properly (restart the machine and log out and back in).

If you are unable to submit properly email your school administrator immediately attaching a copy of the submission and let them know you have a problem.  They can then clear the submission and allow you to try again if necessary.