Hey @ChrisH770 ,
If you are curious to know how it happened the only option you have is to get a ticket opened with Microsoft, they can run a deletion report on your mailbox and can tell you what client (mobile/Desktop) was used to delete the emails, though Microsoft wont share the report with you due to compliance reasons but they can surely tell if the emails were deleted via a client or if office 365 system deleted it due to any retention policies etc.
As far as recovering the email is concerned, check recoverable items in outlook, the emails should be present there. If you cant find them in recoverable items even, you can also run a ediscovery search with date filters to export a PST. Once you have the PST you can import it back again via outlook.
Ediscovery in office 365: https://docs.microsoft.com/en-us/microsoft-365/compliance/content-search?view=o365-worldwide
Thanks
