User can no longer sign in

Why is that?
Some people say they have their login information saved on their computer and should be logged in automatically.
And when they try to send a new password to their mail it says there's no such email address registered.
And deleting their cookies and cache doesn't help.

They've made new users and they can use those just fine.

Could be a browser issue, that their saved information was lost.

How can this be fixed then?

Try using a different browser