Dear _____ (name):
In the past I have valued your patronage. But despite
the fact that we have always enjoyed a mutually pleasant and
profitable relationship, you have not placed an order with me
in recent (time period). Am I responsible for something said
or done that offended or displeased you?
Please let me know if anything is troubling you that
I can rectify. Just call on the phone so we can talk. I
definitely respect your insight and good will.