@Pietro185 how terrible for you and as importantly for your poor neighbours
As a highly experienced superhost, I am sure you are familiar with Airbnb's guarantee? You need to contact them immediately and put in a claim. (see Airbnb Help Centre for details). This needs to be done before your next guests arrive (but sure you can't let it in this state anyway).
Hopefully you have photos and video evidence of the all the damage.
You will need to get estimates for your repairs and repairs in neighbouring flats.
You can also claim on your own home specialist insurance for short term lets.
By the way how were they able to bring in extra guests? Do you not have CCTV or similiar to monitor those coming into and leaving your property?
And when you let the place as a whole listing you state clearly that guests have access to both bathrooms (presume this must have been a whole listing let or you would have seen them bring in extra guests/damage the floor/flood the bathroom.
Good luck.