Nothing. There is nothing you can do except check your cancellation policy. Do you have a Moderate policy or Strict? If so, then you will get paid a portion of the booking cost. MIne is now STRICT because I had a guests tricked me into changing their dates so they could get their monies back. Be aware of that as well. But for now, if you have any REAL concerns about this, you can always contact Airbnb if you think you really need to.
You might want to ALSO consider contacting the guests to make sure they are ok. It shows you care. I know I would. Maybe they accidently did that. I have had guests that accidently did something to their booking. Its always good to check. But keep in mind, you can now accept another booking for those dates since the guests did cancel. Good Luck.
If you feel that me or another host have helped you, feel free to click on the "thumbs up" button at the end of any post. Thank you so much.
Aloha, Momi
Great way to contact Airbnb or via Twitter at AirbnbHelp / Facebook