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one needs to look at what happened exactly to know if you can get them back or not?

If the users were disabled they can be enabled again.
If they were deleted you have to recreate them again.
Log into the admin portal > click on More > and check the audit trail to see what happened to the users.

if you can't figure it out by yourself, your best option is to contact support. They should be able to see everything that has happened to the account.

Btw you can also check Admin portal > Billing > Licenses & Inventory.
Here you can see the numbers of Licenses you have (do you have less licenses than you have users?) this means licenses were deleted?
Do you have a bunch of licenses that are Available and not in use?