Dear {Recipient},
Please accept my sincere apologies for cancelling our plans to {do activity} on {date}. I was really looking
forward to it and cancelling was the last thing on my mind. Unfortunately, {reason for cancelling} and I had no choice but to {be somewhere else/stay at home}.
I hope we can still make our original plan happen. Let's find a time and date that works for us bothmaybe sometime next week? Please let me know your availability.
Respectfully,
{Your Name}