First, I'm glad you're physically unharmed.

Have you notified/contacted your employer? I don't know if it's contract-specific or not, but for us, any post-rental conversations about liability/damage bypass the renter and go straight to the company. (That may change in a theft vs. simple damage situation, but I thought I would at least point it out. Your company may also not want you to speak directly with National since National has already retrieved the vehicle.)