What would you say to change an upset customer's mind?

I got a new job and tried too hard to please a big client only for it to backfire and cause her to cancel her order. Now my job is on the line. She sent a letter to my boss saying I've caused drama and strife etc. I had sent her a detailed but urgent sounding letter urging her to follow up with things that took a lot of time for me to write up but rubbed her the wrong way with the writing and her interpretation. My boss won't let me talk to her and is thinking of what he should say to her. Do you have any ideas?