In qbit click on the stalled torrent and look for “last time seen complete” if it says “never” it means the file is dead. None of the seeders have the last bit of the file and the seeders are other people waiting for somebody to complete the file. If this happens just delete and restart from a different source.
Edit: This applies to files that have started downloading then stalled out later. If your download is still at 0% you have a separate issue.