Lawyer’s letter says don’t destroy ex-worker’s records—now what?

Q. My department recently received a “litigation hold letter” from an attorney’s office. It instructed us not to delete or destroy any documents belonging to a former employee of ours ... While we have a few documents related to this person’s employment, the significant majority of her personnel documents were destroyed through our normal record-retention process. Are we required to comply with this litigation hold letter ...? If so, what can we do about the documents already deleted?