My apartment has just had a major flood. I have guest coming this weekend but I know it won't be ready in time. It probably won't be ready for weeks. Do I just cancel the upcoming reservations or do I contact airbnb?
Definitely contact Airbnb, as this sounds like what they call "extenuating circumstances." A cancellation that you make on your own will incur financial penalties to you, and those dates will be permanently blocked on your calendar. If you have trouble reaching a live voice, I've had some luck reaching support by sending a private message through Twitter, which seems to be their contact of choice.
@Megan40 follow @Jude7's advice. I had a gas leak the very day an international traveller was to arrive. Airbnb took care of contacting and redirecting the guests and waived all penalties for my having to cancel. Get your documentation in order because Airbnb will ask for it.
My current guest had a flood in the basement and I have to cancel the upcoming guest. I need AirBnB to help get them resorted and to help with the clean up of the apartment.