I'm of the older generation, so to me a personal letter would say you really wanted to reach me to explain things. You could make it a card with a short apology and also an invitation to dinner or something? Make it specific...like next Friday, the xx of Dec. at 7:30 at joe blows restaurant....That would make me at least want to respond. Put your phone number and ask for a response. Good luck...let us know what happens.