Hi Alejandra,
> If contact checked "Do not email" option.
Change Bulk Email field of contact to "Allow".(In Contact > Details > Contact preferences)
> If contact unchecked checkbox of a subscription list.
Add contact to a specific subscription list/marketing list again.(Site map > Customers > Subscription lists)
Note: 1. "Do not bulk email" overrules subscription list memberships, but it also means that list memberships will be reactivated if a contact later clears "do not bulk email".
2. We may need to get confirmation/consent from contacts to manually change their already chosen preferences.
Regards,
Clofly