RE: mailbox becomes empty after migrating to office 365

Hi Long,

First, I would like to confirm some information below:
1. Which migration type did you use, staged migration, cutover migration or IMAP migration?
2. Are there any errors about this failed mailbox?

According to your description, if only one mailbox failed to be migrated to Office 365, I suggest you use PST files to migrate them.

Here is an article about PST files:
How to manage .pst files in Microsoft Outlook

Thanks,
Kid Yang