Short answer
HubSpot's KB: if a contact was unexpectedly unenrolled, re-enroll them manually from the sequence's summary page. The inbox used for the original enrollment must still be connected. The enrollments page shows each contact's status and latest step.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Enrolling through workflows needs Sales or Service Hub Enterprise.
3. Related
Sequence unenrollment settings.