@Anonymous
Yes, this happened to me once. Pretty similar situation to @Ellie331 , except that I was not expecting a deposit nor rental agreement, just my standard requirements that the guest provides a check in time in advance and confirms that they have read the house rules.
This guest booked and then went completely silent. Never responded to a single message, text or call after that and Airbnb couldn't get hold of her either. I was worried something might have happened to her, but since she was unresponsive since booking, I reckon she just had a change of plans, realised she wasn't going to get a refund and so didn't bother cancelling. Would have been nice though to know she wasn't coming.
I didn't cancel the reservation. As @Helen3 said, @Ellie331 has enough grounds for getting a penalty free cancellation from Airbnb but only IF it was an instant booking. However, she would lose the full payment, which I don't think is fair at this late stage.
Anyway, I got paid in full and never heard from the guest again.