A "lost" client is often a misnomer applied when a client or customer is no longer using you or your services. You didn't "lose" the client - they are not like a towel you left at the beach. You know where they are; so they're not "lost."
My mother often told the story of the time her two-year-old, Jeffery, got "lost" at The Jewel. I had to wait until I was a parent to break the news to my mom that she may have had something to do with me getting lost.
There are a number of reasons, some good and some bad, as to why a client chooses to go elsewhere, such as selling their business, outgrowing your capabilities, unresolvable issues, or a lack of perceived value.
No matter what the reason, make sure you understand what happened and why. Educate the entire team on developments in the relationship and communicate positive and negative feedback. Continue the relationship, we never truly say goodbye to our clients, they'll always be a part of our "team" and often come back to work with us as they move companies, start new companies, or refer business or employees.
Relationships are what make the world go round - it's true professionally and personally. Never take them for granted, give before you get, give more than 100% and be open to saying goodbye if things just aren't right. However, avoid using the word "lost" as relationships can always be found again.
"Be kind whenever possible. It is always possible." - Dalai Lama