As someone mentioned earlier, the entry in you database/schema is pointing to a file that cannot be seen or does not exist. If this happens for all users, make sure that your repository path still has room for files AND that the service account that is running QC has the required privileges to create/add files.
If this doesn't resolve the issue, please turn your log level up to debug, add a file to a defect (make note of the exact time!), then set your log level back to its previous settings. Check the log for that time, and if that doesn't provide the answer, please post the relevant section of the log as an attachment for others to analyze.
