That suggests you are sharing the user directories between the two systems, but you have not said which user system you are using.
If you want to "restore" them, the only option will be to restore the last backup you took of the user-directory system you are using.
You could also attempt to repair it - admin accounts can always log in, and there was one created in the internal directory which you can use - you should be able to use that to see what groups have "can log in" (if none, then you can add new ones) and start putting users into the login group.
Catch up with Atlassian Product Managers in our 2020 Demo Den round-up! From Advanced Roadmaps to Code in Jira to Next-Gen Workflows, check out the videos below to help up-level your work in the new ...
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