@Anne-and---Florin0
No, you don't have to refund them - it's completely up to you. My thought would be, what would you like to happen if you were in that position.
We have had a couple of things like this happen over the past year and a half - flights cancelled, etc... We usually give back about 50% of the night's rate - but we make the offer before they even have time to think about it, and they tend to be pretty chuffed that you offer.
Good luck arguing the toss on this between you!