Recovery of mail in Inbox

I recently/yesterday attempted to export a large volume of emails (approximately 50,000) from my Inbox in Mac Mail to a designated folder. The exported file from this attempt shows as zero bytes, and my original Inbox emails have disappeared. I found a separate .mbox file from 2022 that is approximately 1.5 GB, but when I attempt to import it, it only appears as an attachment in a newly created email, and I am unable to open or access the content within. Could you help determine why the export failed, how I might recover the missing Inbox emails, and whether I can retrieve any data from the zero-byte file or the inaccessible 1.5 GB file?