Give a call to whatever agency sent you the letter & try to get an understanding of what regulation or law they think you violated. Ask for the statue number or how you can get a hold of a copy. See if you agree - if so, ask them what you can do to get on the right side. If you don't think your listing violated anything, ask what you need to do to appeal or explain your case.
You have to check in with every level of government/regulation you live under - landlord, HOA, City, County, State - to be sure you're operating legally. The city clerk was very nice and well meaning in telling me I didn't need to do anything on a state level - that the city permit was enough - she wasn't right though. Look through the host responsibilities to see if there's any other spot you overlooked before you started hosting:
https://www.airbnb.com/help/responsible-hosting
And not serving breakfast doesn't necessarily exempt you - in my area ANY person renting up to 3 bedrooms requires a license, whether you serve breakfast or not. It's great to hear everyone's experiences here, but other states' laws don't matter much in Florida. Do your own research.