Shared mailbox disappeared from Exchange Online (still showing On Prem)

[RC1_GL]

We run Hybrid and when someone leaves the company, their user mailbox is converted into a shared mailbox.

I've had it happen twice now where the shared mailbox disappears from Exchange Online but I can still see it in the list on premises. Anyone who had access to the mailbox then can't get into it.

The only thing I can think is different is that these users may have already been moved to our leavers OU and disabled at the point the mailbox being converted. But this is a total guess.

Any tips on how I can get the mailbox to show up in Exchange online again?

Many thanks

[#2]

There is a good chance that someone mistakenly disabled the user account.

Can you check the soft deleted and inactive mailboxes:
Get-Mailbox <Mailbox Name> -SoftDeletedMailbox
Get-Mailbox <Mailbox Name> -IncludeInactiveMailbox

You can also check the deleted MSOL user to check if you find the user

If you find the user in any of these locations you can restore it from here
Undo-SoftDeletedMailbox
Or if you want to create a new shared mailbox and restore the data to it you can run a restore request. New-MailboxRestoreRequest -SourceMailbox <Soft Deleted Exchange Online mailbox> -TargetMailbox <Existing Exchange Online mailbox> -AllowLegacyDNMismatch