@Amy2398
Welcome,
My guess is nobody knows the reason just a bug in the system,
I noticed a few days ago they unblocked a day that I did have blocked because I dont want to do a same day turn over, Nothing I can do now.
All you can do is message the guest back and explain what has happened give them time to find an alternative listing.
You wont get demoted by Airbnb they are not that bad and also message Airbnb whats has happened.
For future reference, always keep checking you listing certain things do happen just when you least expect them to.
Good Luck