What to do - Book keeper withholding Company Accounts

We hired a book keeper from a local advert we placed for our small construction company. They worked for 6 months, then left suddenly for 'personal reasons'. They resigned by email and refused to do a handover. We have discovered that we have no company records on Sage for the last 6 months - no VAT records or nominal ledger, TB. We presume they have this on their home computer but they will not forward a copy to us or answer phone calls or emails. What can we do to get our company records - if they exist at all?