We have migrated all of our users to Office 365. In the last week, I've had two (One an E3 user and one an E1) who have been unable to log into their account. When I checked, both of them were unlicensed. They have been using this fine for at least a couple of weeks, so I know that the licensing was OK at some point. We put their licensing back and they are OK now. They have not been deleted or had their accounts disabled.