Dear
It has been so long since we have had the opportunity to
be of service to you, that I have begun to wonder if
perhaps, we have offended you in some way in the past.
If this is the case, I would greatly appreciate knowing
what happened.  In fact, if you have any grievance with
our firm, I wish that you would call so that we might
discuss the problem.
We have introduced many innovations into our product line
since the last order you placed with our firm, and if the
reason we haven't heard from you has nothing to do with a
complaint, we would appreciate having the opportunity  to
show you these innovations.
In either case, it would be wonderful to hear from you.