Hi Martine,
I think you should apologize in your response. No matter what happened before, it is not fault of the new renter who came to your place and did not get the keys. I would have called several times to make sure they get in because this is the least an Airbnb landlord should do - they provide a place for guests and the guests should gain access for the time they booked.
I can understand your frustration, and here I guess you mixed two things together:
1. The guests did not get the key and had to spend a night in a hotel instead - this is really horrible experience. I would have got really mad, too. So I think you really have to apologize for that and offer them compensation.
2. The guests did not find your place nice, either. Well this could be also caused by the fact they didn't get in at the first place. But there is no point arguing if this is subjective from your renter. They are very unsatisfied and you bear 100% responsibility (even if the last renter made mistakes, you are the landlord who has to make sure things work out.) If you can't do anything else to make them feel a little happier, at least apologize and let them know they have the right to be mad.
These are only my personal opinion, though...
Good luck! (by the way I'm new to Airbnb, too and I just posted something new, I guess mine was also quite maddening, hope this doesn't happen a lot!)
best regards,
Chi