Restore grades: Student accidentally unenrolled from a course

To restore the grades you will need to...

Go to administration -> grades -> general settings, I checked "recover grades default".
Open a new self enrolment  with a new password and make the student enroll again.
Once the student is enrolled back in the course, go to every quiz in the course, select the student name, and click "regrade selected attempts".



* I got inspired here