a section 8 tenant rufuse to pay rent this month

I live in Virginia. Rent a house to a section 8 at 4 month ago. City housing pay $700 and tenant pay $300. She paid on time in the first 3 month. This month she said she has no money. What should I do at this point? Please help, especially Miat42 and VA Landlord. I always read your articles.
Thanks a lot!