Have you ever had to respond to a cease and desist letter? I have been accused of trademark infringement when I had no way of knowing.
What should I do?

Just because someone has accused you of something does not mean that you have done anything wrong or are guilty of anything. The most important thing is not to take any action on your own without the help of a lawyer. A response to cease and desist letter must be drafted with legal assistance if you do not want to have further problems. Contact David Nima Sharifi Esq. attorney, he helped me with the same issue last year.