Nationality through Possession of Status
Those who’ve had and used Spanish nationality for over 10 years continuously, based on a civil registry record, are entitled to that nationality provided they have acted all those years in good faith. Cancellation of that civil registry record does not affect their nationality.
They do however need to have been active users of their Spanish nationality, ie regarded themselves as Spaniards and exercised the rights and fulfilled the duties it brings.
Where to apply?
The application needs to be presented at the Civil Registry of the place where the applicant resides in Spain. Should the applicant reside abroad, they need to apply in their local Spanish Consulate.