Hi. @julie3 ,
Are the emails that were originally associated with your course school-provided emails, or did they use their own home emails?
If your school pays for canvas, part of me wants to say your administrator locked new enrollments on the course, and in the process those users were removed. Does your course have a set end date on it that would restrict users from viewing the course after a specific date? This would be visible within the course settings.
Edit: I just saw you use free for teacher, my apologies! Since your canvas is not a paid for edition, I recommend reaching out to canvas support for assistance, especially if there isn't an I.T. that would have canvas access with admin-level permission to fix this issue.
Hope I'm able to help out,
Noah